Microsoft has it all explained here and the information below is just really basic summary of it https://dotnet.microsoft.com/learn/dotnet/what-is-dotnet

.Net is a free opensource cross development platform, which is used to make a variety of applications(so things for the web,mobile,desktop,gaming,and IoT(which stands for Internet of things)). It allows you to use multiple languages, editors and libraries to build said applications. The actual languages that these applications can be written in is C#, F#, and visual basic. The application will work on any compatible OS, but it depends on what .net implementation you are using.

The .net framework is for the stuff on windows.

Xamarin/Mono is for the major mobile operating systems.

THe .Net core is for implementation for windows,Linux, and MacOS.

.Net standard is the common API’s for all .net implementations, with the specific ones often having additional API’s like the framework has additionall API’s for accessing the windows registry.

NuGet is the package manager for .net.

.net is opensource under the .Net Foundation, which is a seperate independent organization.

A lot of people use .net because it is open source, it has a managed run time which improves security, it’s fast, it’s cross-platform, and it has shown to be the most productive platform for developers.