The .NET Standard Library is a set of APIs which are implemented by a .NET runtime.
More formally, it is a specification of .NET APIs which make up a uniform set of contracts that you compile your code against. These contracts have underlying implementations for each .NET runtime. This enables portability across different .NET runtimes, making it so that your code can effectively "run everywhere".
The .NET Standard Library is also a build target, where it is known as the .NET Standard. You can currently target .NET Standard 1.0-1.6. If your code targets a version of the .NET Standard, it is guaranteed to run on any .NET runtime which implements that version.+
To learn more about the .NET Standard library and targeting the .NET Standard, see .NET Standard Library.
.NET runtimes
There are 3 primary .NET runtimes which Microsoft actively develops and maintains:
.NET Core
.NET Framework
and Mono for Xamarin.
.NET Core
.NET Core is a cross-platform runtime optimized for server workloads. It implements the .NET Standard Library, which means that any code that targets the .NET Standard can run on .NET Core. It is the runtime used by ASP.NET Core and the Universal Windows Platform (UWP). It is modern, efficient, and designed to handle server and cloud workloads at scale.
.NET Framework is the historical .NET runtime that has existed since 2002. It is the same .NET Framework existing .NET developers have always used. It implements the .NET Standard Library, which means that any code that targets the .NET Standard can run on the .NET Framework. It contains additional Windows-specific APIs, such as APIs for Windows desktop development with Windows Forms and WPF. .NET Framework is optimized for building Windows desktop applications.
Mono is the runtime used by Xamarin apps. It implements the .NET Standard Library, which means that any code that targets the .NET Standard can run on Xamarin apps. It contains additional APIs for iOS, Android, Xamarin.Forms, and Xamarin.Mac. It is optimized for building mobile applications on iOS and Android.+
docs.microsoft.com: .NET Platform Guide
.NET Architectural Components
Framework versions
.NET Platforms
.NET Standard Library
The .NET Standard Library is a set of APIs which are implemented by a .NET runtime.
More formally, it is a specification of .NET APIs which make up a uniform set of contracts that you compile your code against. These contracts have underlying implementations for each .NET runtime. This enables portability across different .NET runtimes, making it so that your code can effectively "run everywhere".
The .NET Standard Library is also a build target, where it is known as the .NET Standard. You can currently target .NET Standard 1.0-1.6. If your code targets a version of the .NET Standard, it is guaranteed to run on any .NET runtime which implements that version.+
To learn more about the .NET Standard library and targeting the .NET Standard, see .NET Standard Library.
.NET runtimes
There are 3 primary .NET runtimes which Microsoft actively develops and maintains:
.NET Core
.NET Core is a cross-platform runtime optimized for server workloads. It implements the .NET Standard Library, which means that any code that targets the .NET Standard can run on .NET Core. It is the runtime used by ASP.NET Core and the Universal Windows Platform (UWP). It is modern, efficient, and designed to handle server and cloud workloads at scale.
To learn more about .NET Core, see the .NET Core Guide.
.NET Framework
.NET Framework is the historical .NET runtime that has existed since 2002. It is the same .NET Framework existing .NET developers have always used. It implements the .NET Standard Library, which means that any code that targets the .NET Standard can run on the .NET Framework. It contains additional Windows-specific APIs, such as APIs for Windows desktop development with Windows Forms and WPF. .NET Framework is optimized for building Windows desktop applications.
To learn more about the .NET Framework, see the .NET Framework Guide.
Mono for Xamarin
Mono is the runtime used by Xamarin apps. It implements the .NET Standard Library, which means that any code that targets the .NET Standard can run on Xamarin apps. It contains additional APIs for iOS, Android, Xamarin.Forms, and Xamarin.Mac. It is optimized for building mobile applications on iOS and Android.+
To learn more about Mono, see the Mono documentation.