I have read about the difference between .NET Standard and .NET Core, but I really don't know what the difference is, or when to choose a .NET Standard library project and when to choose a .NET Core library project.
I have read that .NET Standard is to ensure that a set of APIs are always available, no matter the platform used (as long as that platform is compatible with the .NET Standard version that I have chosen). If I'm not mistaken, this means that I can create a class library of .NET Standard and then use it on any platform that is compatible with the .NET Standard version that I have chosen.
With .NET Core, I have read that it is intended for cross-platform use too, so if I choose a .NET Core library it seems that I can use it on many platforms too, just like .NET Standard.
So at the end, I don't see the difference. When should I use which? What is the difference between them?
I will try to further clarify your doubts and extend Jon Skeet answer.
.NET Standard is a specification, so a library compiled for a specific .NET Standard version can be used in different .NET Standard implementations.
As said in my other comment, a good analogy for the relationship between .NET Standard and other .NET Standard Implementations (.NET Core, .NET Framework, etc) is this gist by David Fowler: .NET Standard versions are Interfaces, while frameworks are implementations of those interfaces.
This simplified diagram may help to understand this relationship:
Anything targetting NetCore10 has access to INetStandard15 APIs and NetCore10 specific APIs (such as DotNetHostPolicy).
Of course this library cannot be used in different INetStandard15 implementations (NetCore10 is not convertible to NetFramework462 or Mono46).
If you, instead, need access only to INetStandard15 APIs (and target that specification instead of a concrete framework) your library may be used by any framework which implements it (NetCore10, NetFramework462, etc.)
Note: in the original analogy David Fowler used interfaces for both .NET Standard versions and frameworks implementations. I believe that using interfaces and classes is, instead, more intuitive and better represents the relationship between specifications and concrete implementations.
.NET Core is an implementation of .NET Standard. It's available on multiple operating systems, but that's not the same thing - there are other implementations of .NET Standard as well.
So if you create a .NET Core library, it will have access to things that are implemented in .NET Core, but aren't part of .NET Standard, and your library won't be compatible with other implementations of .NET Standard, such as Xamarin, Tizen, full .NET desktop framework etc.
In short: to achieve maximum portability, make your library target .NET Standard.
.NET Core Class library is basically subset of .NET Framework library, which just contains less APIs. Sticking to .NET Core Class library makes difficult to share code between runtimes. This code might not work for different runtime (Mono for Xamarin), because it doesn't have the API that you need. To solve this there is .NET Standard, which is just set of specification that tells you which APIs you can use. Main purpose of .NET Standard is to share code between runtimes. And important that this specification implemented by all runtimes.(.NET Framework, .NET Core and Mono for Xamarin).
So if you sure that you will use your library only for .NET Core projects, you can ignore .NET Standard, but if have even tiny chance that your code will be used by .NET Framework or Mono for Xamarin then better stick to .NET Standard
Also note that higher version of .NET Standard contain more APIs, but lower version supported by more platforms. Therefore if you create .NET Standard library that you want to share between runtimes then target the lowest version you can, which help you reach maximum amounts of platforms. For example, if you want to run on .NET Framework 4.5 and .NET Core 1.0, the highest .NET Standard version you can use is .NET Standard 1.1. This this great table from documentation for more info about it.
PS: Also if you want to convert you library to .NET Standard, .NET Portability Analyzer could help you with that.
.NET Standard is a specification of APIs that all .NET implementations must provide. It brings consistency to the .NET family and enables you to build libraries you can use from any .NET implementation. It replaces PCLs for building shared components.
.NET Core is an implementation of the .NET Standard that’s optimized for building console applications, Web apps and cloud services using ASP.NET Core. Its SDK comes with a powerful tooling that in addition to Visual Studio development supports a full command line-based development workflow. You can learn more about them at aka.ms/netstandardfaq and aka.ms/netcore.
The above, together with a very clear explanation of most of the stuff discussed in this question can be found in the following extremely helpful article by Microsoft (MSDN - September 2017): .NET Standard - Demystifying .NET Core and .NET Standard
.NET Standard is a specification of .NET APIs intended to be available on .NET implementations. This enables to define uniform set of BCL APIs for all .NET implementations.
.NET Core is one such implementation of .NET Standard. .NET Framework is another implementation of .NET Standard.
Image from .NET Blog
Federicos answer gives you a graphical overview of how each framework evolve with versions. Take a look at below diagram from Microsoft Docs.
Targeting .NET Standard increases your platform support whereas targeting a particular .NET platform such as .NET Core (or .NET Framework) will allow you to use all the platform features for that platform.
Did you mean .NET Framework? Because .NET standard is an implementations, such as .NET Framework, .NET Core and Xamarin.
I love .NET Core because we can host it on Linux (use nginx in my experience). It's different than .NET framework which is you can only host on IIS. You can consider about hosting budget in this case (Because windows server is expensive for me).
In the development environment perspective, .Net core is lightweight. So, you can use VSCode, Sublime, for IDE (not only visual studio).
In simple terms, .NET standard is used for writing class library projects which compiles to dll. .NET Core can be used for developing actual web applications which can run on all operating systems (Windows, Linux, MacOS). (In .NET Core 3 Microsoft has provide the functionality to develop desktop apps using WPF, but uptil now these apps will not be cross platform and will only run on windows system. In future Microsoft might make them cross-platform too) .NET standard libraries/dlls can be used in any application which uses .NET (.NET framework, .NET Core) which means that you can use .NET standard with both .NET framework and .NET core.
来源：https://stackoverflow.com/questions/44085424/net-standard-vs-net-core