.Net Framework vs .Net Core vs .Net Standard

What is .Net Standard and why does it show as a project platform type in Visual Studio when I click New Project?
Well, .net Standard is NOT a platform, it is a standard. If we remember that, then it makes sense. Think of .net standard as the mesh that maps the various platform version together for interoperability. See the below table:

What is .NET Standard?

.NET Standard is a specification (not an implementation of .NET) which defines the set of APIs that all .NET implementations must provide. It addresses the code sharing problem for .NET developers across all platforms by bringing APIs across different environments.
We can think of it as another .NET Framework, except that we use it to develop class libraries only. .NET Standard is a successor of the portable class library.
Ok, so .NET Standard specifies the APIs that need to be implemented.
But which APIs does it cover?
To answer this question in short – there are multiple versions of .NET Standard. Each version includes a set of APIs which we are going to cover in a while.