The .NET framework has been branched into several implementations to match the constraints of the target platforms, e.g. Compact Framework for Pocket PC, Mono for Linux and the full .NET Framework for Windows.
This poses a problem regarding library compatibility between them. Writing a library is not straightforward since one has to target the specific framework in which the library is consumed. This has somewhat been alleviated by Portable Class Libraries (PCLs), since this type of library can target multiple frameworks. It is not a 100% solution, since when a new framework emerges the library in question is not automatically compatible with the new framework.
The solution has been to introduce a versioned formal API specification called .NET Standard. This standard is implemented by all the most common framework implementations. Writing a library targeting .NET Standard 1.0 ensures that the library can be consumed across different frameworks, if they implement the .NET Standard 1.0.
The .NET Framework 4.5 implements .NET standard 1.1 while .NET Framework 4.6 implements .NET Standard 1.4. Writing a library for both .NET Framework 4.5 and 4.6 can easily be done by targeting .NET Standard 1.1. Also, Windows Phone 8.1 and Mono implements .NET Standard 1.3 or above making the said library compatible with these implementations.
Each new version of the .NET Standard inherits from the previous version, making a new version a super-set of the old version. The API specification is written i C#, making it straight forward to read and adapt the API to a new implementation.
.NET Core is the latest framework implementing .NET Standard, more on this later.