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What’s the difference between NETStandard.Library package and Microsoft.NETCore.App packages?

I wanted to know the difference between the two packages and if it had anything to do with .NETCore and NETStandard and compatibility. I found this:
The NETStandard target framework is an abstract target framework that represents API surface of many frameworks and platforms. As such NETStandard assemblies can run on any platform that supports the NETStandard targeted by that assembly, for example: .NET Desktop, Windows Phone, Universal Windows Platform applications, .NET Core applications, etc. NETCoreApplication is a concrete target framework that represents a single platform with both API surface and implementation. .NET Core applications are runnable on their own. .NETStandard libraries must be published-for or consumed-by a specific concrete target framework to be used in that type of application.
on https://masteringalm.github.io/framework/2018/07/24/NetStandardvsNetCoreApp-Project-Types.html
But I’m not really sure how to use this to describe the differences between the two packages. I saw a post on this link: What's difference between .NetCoreApp and .NetStandard.Library?
Where it said:
NetCoreApp is a platform and .NetStandard.Library is a library supposed to be cross platform (portable class library) for various .NET platforms runtimes.
I think this connects to differences between the two specific packages mentioned in the question header, but could anyone describe the differences between the packages in a not so convoluted way?

.net standard
Each implementation of the managed framework has its own set of Base Class Libraries. The Base Class Library (BCL) contains classes such as exception handling, strings, XML, I/O, networking, and collections.
.NET Standard is a specification for implementing the BCL. Since a .NET implementation is required to follow this standard, application developers will not have to worry about different versions of the BCL for each managed framework implementation.
Framework Class Libraries (FCL) such as WPF, WCF, and ASP.NET are not part of the BCL, and therefore are not included in .NET Standard.
The relationship between .NET Standard and a .NET implementation is the same as between the HTML specification and a browser. The second is an implementation of the first.
Hence, the .NET Framework, Xamarin, and .NET Core each implement .NET Standard for the BCL in their managed framework. Since the computer industry will continue to introduce new hardware and operating systems, there will be new managed frameworks for .NET. This standard allows application developers to know that there will be a consistent set of APIs that they can rely on.
Each .NET version has an associated version of the .NET Standard.
.NET Core is a free, cross-platform, open source implementation of the managed framework. It supports four types of applications: console, ASP.NET Core, cloud, and Universal Windows Platform (UWP). Windows Forms and Windows Presentation Foundation(WPF) are not part of .NET Core.
Technically, .NET Core only supports console applications. ASP.NET Core and UWP are application models built on top of .NET Core.
Unlike the .NET Framework, .NET Core is not considered a Windows component. Therefore, updates come as NuGet packages, not through Windows Update. Since the .NET Core runtime is installed App-Local, and applications are updated through the package manager, applications can be associated with a particular .NET Core version and be updated individually.
For more information visit https://www.infoq.com/news/2017/10/dotnet-core-standard-difference/

Overly simplistic, but think of .NET Standard as a library that runs on any platform that can run .NET. This means it can run on Linux, Mac, Windows, etc. .NET Core is a minimalistic version of the "framework", so you can keep your your apps as small as possible and only add what you need. Core means only the core functionality that is absolutely required for an app to run.
Why is standard and core different? It is largely because the functionality of .NET on other platforms started open source with someone other than Microsoft. As such, the features have lagged behind .NET. The name of the .NET "framework" on other platforms was Mono. As some point .NET Core may well replace standard, but certain things have to get to the same level.
I imagine some purist will disagree with my oversimplification, but it is an easy way to get your head around the libraries.
As for what specifically is different between the two (or "what is missing from standard to be equivalent to core") - I am not sure. I am sure it can be looked up. In many cases, I have found Core works fine, such as dockerizing the application (Yeah, I made the word "dockerizing", but meaning running in a Docker container). The problems are when you run some package that cannot be handled in "Mono".
When it comes to packages you add into the standard or Core (Nuget anybody?), where there is a difference deals with one having features that cannot run in standard.

How to decide .net framework versions to create a library [duplicate]

This question already has answers here:
What is the difference between .NET Core and .NET Standard Class Library project types?
(13 answers)
Closed 3 years ago.
I want to create a library to use in .Net framework applications and Xamarin applications. But there are 3 framework versions:
.net core
.net framework
.net standart
So I could not decide which version to use my common library.
If I create a .net standart library, does it work in .net framework and .net core.
If I create a .net core library, does it work in .net framework and .net standart.
I am confused about frameworks.

This might help you decide
.Net Standard : Used for building libraries that can be referenced from all .NET implementations, such as .NET Framework, .NET Core and Xamarin
.Net Core : Used for building cross-platform console apps and ASP.NET Core Web apps and cloud services.
So when if you want your library to be supported by all different type of .Net implementations, .Net standard is the way to go.

As you're writing a C# library I imagine you know the difference between a class and an interface. A class is a concrete implementation of some feature set, and an interface defines what features you can expect from an instance that implements it.
Using this as an example, .NET Framework and .NET Core are like classes. .NET Framework is the "classic" implementation, and .NET Core is a newer implementation which has advantages such as being able to run on Linux. If you build a library to target .NET Core or .NET Framework, you are building it to target one of those concrete implementations.
.NET Standard on the other hand is like an interface. Each version of .NET Standard provides a set of features, and different versions of .NET Framework/.NET Core implement different versions of .NET Standard. When you build a library that targets a given version of .NET Standard, you're saying that you can support all of the concrete implementations in the corresponding column of that table.
Deciding what version of .NET Standard to target will depend on what functionality you need to implement your library. More features usually means a higher version and supporting fewer implementations. Fewer features means a lower version and more widespread support.

So many confusing other answers here.
First and foremost it depends on what platforms you are targeting.
is it a generic use library you want share with the world?
is it an application specific library (i.e. reusable code for the same application, i.e. a line of business application) which is to be used on multiple platforms
Case 1
You should go with .NET Standard, since it gives you the most platforms. Which version you use, depends on the features you need.
If you want reach the most platforms, try to target as low as possible (.NET Standard 1.0, it targets .NET Core 1.0, .NET Framework 4.5, Mono, Xamarin iOS 10/Mac 3.0/Android 7.0, UWP 10 and Uniy 2018.1 and all newer versions of these).
You can see the exact .NET Standard Matrix in the provided link.
If you need specific API you have to target a higher version, such as .NET Standard 2.0 which a lot (~22k new APIs from .NET Framework have been ported to .NET Core 2.0 from 1.1) APIs than .NET Standard 1.1.
This may not allow you all APIs (no WPF/WinForm specific apis) but in generic use reusable libraries this shouldn't be an issue.
Case 2
Here, you can also apply Case 1 tips, if possible.
If that doesn't cover your required Apis and you know you don't want target .NET Core or Unity, you can still use the old styled PCLs: Portable Class Library.
They are a more complicated versions of .NET Standard (kinda a predecessor of .NET Standard), where depending on which platform you target the API surface shrinks to only allow APIs to be used which run on all these platforms.
It's not recommended these days to use PCLs, since .NET Standard is preferred and more easy (for library authors) to use and target multiple platforms.
Last but not least, if you really need some feature only on Windows and .NET Framework (or you don't care for .NET Core), you can still cross-target, i.e. have a .NET Standard 2.0 for all platforms and add specific APIs only to net45 target and preprocessor instructions (#if NET45/#endif).
This compiles into two libraries, one for netstandard2.0 and one for net45 (.NET Framework 4.5).

C# build generates a lot build-in dll files

I am writing a simple .NET 4.6.1 console application (not .NET core), as below
References are
The problem is, once compiled, it generates a hefty lot of dlls that are supposed to be build-in dlls. What is the problem?

You left a big clue in the screenshot.
They're recommending that libraries target .NET standard.
There's a good discussion at:
https://github.com/dotnet/standard/issues/146
Libraries should generally target .NET Standard as this ensures that
they can be consumed by any app. There will be circumstances where you
need to access .NET Core specific APIs, either because the API is new
and not implemented anywhere else, or the concept is .NET Core only.
That's why I believe we should make it easy to retarget between .NET
Standard and .NET Core so that developers never have to fear being
"locked in". Start with NET Standard and retarget if necessary &
revert back once a new version of the standard is available that has
all the APIs you need.
So gRPC now works with .NET Standard.
If you don't want to opt-in, you could build the particular library targeting the .NET Framework.

What are the platforms in the .NET Platform Standard?

Currently trying to learn about the .NET Platform Standard I've found myself quite confused about the idea of "different platforms".
I'll try to make my point clear. What I currently now about the .NET Framework is that .NET is roughly speaking made up of the CLR, the BCL and supporting software to boot the CLR and provide the interface between the virtual machine and the underlying OS.
So when we code using the .NET Framework we indeed target some version of the framework because the types we are using from the BCL come with the framework and so depend on the specific version.
Now, .NET Core is quite different as I understood. It is not all packed together like that. We have the CoreCLR which is a lightweight VM to run the IL, the CoreFX which are the libraries properly organized as NuGet packages and we had up to now the DNX/DNVM/DNU which provided the supporting stuff like booting the CoreCLR and interfacing with the OS.
Anyway, despite if we install the framework on Windows 7, Windows 8 or Windows 10, we code against the framework.
Now, on the .NET Platform Standard spec we see the following definition:
Platform - e.g. .NET Framework 4.5, .NET Framework 4.6, Windows Phone 8.1, MonoTouch, UWP, etc.
Also we see after that a list of platforms, which includes
.NET Framework 2.0 - 4.6
Windows 8
Windows Phone 8.1
Silverlight 4, 5
DNX on .NET Framework 4.5.1 - 4.6
DNX on .NET Core 5.0
Now this confuses me completely. I always though: we code against the .NET Framework and the framework is the framework no matter what.
But here we have these platforms which includes the .NET framework as just one of many platforms. We have for example Windows 8, but wait a minute, running .NET on Windows 8 is not just the same thing as running .NET on any other OS? Why it is separate from the .NET Framework 2.0 - 4.6 platform?
We also have DNX as a specific platform. This makes me wonder: the platform is that "supporting stuff" related to booting the Virtual Machine and providing the interface with the OS? Or the platform includes the Virtual Machine?
Anyway, as can be seen I'm quite confused. What are those platforms indeed and how this relates to my current understanding of the .NET Framework? Also, why .NET Framework 2.0 - 4.6 is described separetely? Isn't everything described here some version of .NET Framework unless .NET Core?

we code against the framework.
Well, sure you are. When you manipulate strings in your code then you'll always use System.String. And it (almost) always behaves the exact same way with the exact same methods and properties.
But displaying the string does have implementation details that you cannot really ignore:
If you want to show it in a Unix terminal on Linux or OSX then you need to target Mono or CoreCLR, the framework implementations that can run on such operating systems.
If you want to show it in a Windows Store app (aka WinRT, aka Windows 8, aka UWP) then it is actually a HSTRING under the hood, an very well hidden detail that you don't have to worry about. But does require an UI gadget, like Windows.UI.Xaml.Controls.TextBlock, a class that is highly specific to WinRT
If you want to show it in a browser then you need to target ASP.NET or Silverlight, framework hosts that were optimized to run on a web server or as an add-in for a browser.
If you want to show it on a device that is powered by a small lithium-ion battery, like a phone, then you'll inevitably have to deal with a framework version that was optimized to use as little power as possible. That does affect the code you have to write, there is a huge difference between code that burns 100 Watts and code that keeps a tiny battery alive for 8 hours. Nothing you can directly see, beyond the need to use async/await a lot, but certainly something that affected the runtime very heavily. Targeting Xamarin or WinRT is required.
If you want to show it on any operating system then you do need to target a framework version that does not use the kind of tricks that .NET uses on Windows to have an EXE launch the CLR virtual machine. That requires dnx.exe, just like you'd use java.exe or python.exe for programs written in Java or Python.
It would be lovely if those implementation details did not matter. But not the way it works in practice, as .NET proliferates and becomes available on more and more devices and operating systems it inevitably also becomes more convoluted. Choose your intended targets early, it is important.

There are many Frameworks (.NET Framework, WinRT, UWP, Silverlight, .NET Core, Windows Phone, Mono, Micro Framework und the old Compact Framework) not just only the .NET Framework.
The new way is to program against a platform standard which supports one or more of this frameworks. The platform standard defines an API which matches one or more frameworks. This means if your application supports platform standard 1.1 you will probably support almost all frameworks. Platform standard 1.4 will support .NET Framework 4.6.x and .NET Core only
Have a look at this document: https://github.com/dotnet/corefx/blob/master/Documentation/architecture/net-platform-standard.md

Now this confuses me completely. I always though: we code against the .NET Framework and the framework is the framework no matter what.
No, there are actually plural .NET frameworks or platforms as you like to call them. Prior to .NET Standard, you used to target a single framework (maybe the full one, currently at version 4.6.3, if you develop web applications or windows services). DLLs targeting a framework are not compatible with another one. I.E. a DLL developed for the full .NET framework cannot be executed on Windows phone 8.1.
These frameworks actually implement a quite small common set of librairies, but also specific libraries for dealing with the platform they are intended for. I.E. libraries for managing a web server hosted on IIS in the full .NET framework, or functions for dealing with a mobile phone in the windows phone 8.1 framework.
Before .NET Standard was the PCL
There was although a workaround, called PCL which stands for "Portable Class Libraries". By using only the small common subset of methods/assemblies in two or more .NET frameworks, one could develop a library that could be included in projects targeting different frameworks. For instance, PCL profile 37 means you want your library to be usable in .NET Framework 4, Silverlight 5, and Windows 8 projects.
Please have a look at this for a list of PCL profiles and their compatibilities (I don't know if it's exhaustive): http://danrigby.com/2014/05/14/supported-pcl-profiles-xamarin-for-visual-studio-2/
Now what about .NET Standard ?
The goal with .NET Standard is to simplify this and get rid of PCLs. Roughly, .NET Standard defines a contract (a set of classes and methods), that will be implemented by all .NET frameworks. If you develop a library that targets .NET Standard, you're sure it can run on all .NET frameworks. It's the basic idea/goal behind it (even though it's a bit more subtle).
Have a look at this for the exact compatibility: https://blogs.msdn.microsoft.com/dotnet/2016/09/26/introducing-net-standard/#user-content-whats-new-in-net-standard-20
If you look at the compatibility table, you'll see that a library targeting .NET Standard 1.6 is usable as is (no need to recompile it) in .NET Framework 4.6.3 and .NET Core 1.0 applications.
In other words, we can say that .NET Framework 4.6.3 and .NET Core 1.0 both implement the .NET Standard 1.6 contract: its classes and methods.
If you also want your DLL to be usable in a windows phone 8.1 project, you'll have to target .NET Standard 1.2, which offers less functions than .NET Standard 1.6 (no System.Net.Sockets for instance).
See here for a list of available namespaces in each version of .NET Standard https://github.com/dotnet/corefx/blob/master/Documentation/architecture/net-platform-standard.md#user-content-list-of-net-corefx-apis-and-their-associated-net-platform-standard-version

Possible to create an application c# with no framework

for strict constrains in my scenarios, I have very few room to install my application, and .net framework is not installable (any version).
If the application is really simple (more or less), it is possible to create an application in visual studio (in c#) with no dependendency from the .net framework?
Thanks!

Short answer: NO.
There is no way to create a .NET application without any framework.

It is possible to compile a C# app such that it has no dependencies on any of the built-in .NET types & libraries, by using the /nostdlib switch (see http://msdn.microsoft.com/en-us/library/fa13yay7.aspx). You then need to supply your own System namespace.
However, this doesn't remove the need for the .NET framework on the target machine if you use the standard C# compiler. As well as containing the built-in types, the framework also includes the JIT IL compiler, the CLR extra, which all .NET executables and dll's are reliant on.
There are ways of compiling C# code such that it doesn't need the framework though. The Xamarin product for example (http://xamarin.com/), supports compiling C# code to native iOS apps, which are wholely independent of the .NET framework. I'm not aware of any equivalent for "desktop" OS's though.

Writing, compiling and running a C# program without .Net means running a special C# compiler that produces native code instead of managed code. I think such a compiler exists from WinRT for mobile phones, which uses COM instead of .Net (And C++/CX instead of C++/CLI). Code it produces does not depend on the .Net Framework, but does depend on the WinRT runtime.

You may create mono GTK# application and then use mkbundle to generate independent executable. You can use Visual Studio to build your logic and use Xamarin studio to build GTK# GUI. For more information about mkbundle see this and this.

To reply to your query. It really is not possible to create a .Net application without the .Net frame work. And moreover if you have installed Visual Studio by default it would have asked you to install .Net framework or would have installed it by default. In that scenario there is already .Net framework installed in your PC.
Thanks