.NET is a developer platform made up of tools, programming languages, and libraries for building many different types of applications.
There are various implementations of .NET. Each implementation allows .NET code to execute in different places—Linux, macOS, Windows, iOS, Android, and many more.

.NET Framework is the original implementation of .NET. It supports running websites, services, desktop apps, and more on Windows.
.NET is a cross-platform implementation for running websites, services, and console apps on Windows, Linux, and macOS. .NET is open source on GitHub. .NET was previously called .NET Core.
Xamarin/Mono is a .NET implementation for running apps on all the major mobile operating systems, including iOS and Android.