.NET Standard is an interface. It describes a specification that must be met. .NET Framework, Xamarin, UWP and .NET Core are implementations. Both have APIs that are specific to the implementation (for example, .NET Framework has APIs that are specific to Window, while .NET Core is cross-platform and doesn't have them, and Xamarin has mobile-specific APIs that don't make sense on desktop). At a minimum they both implement the APIs designated by the .NET Standard specification. It is a guideline for you to write libraries that can be shared anywhere without recompilation.