C# is used with the .NET Framework, which allows you access to a number of libraries that are specifically intended for use with Windows. The Common Language Runtime (CLR) is the component of the .NET Framework that allows you to compile and execute applications written in either C# or Visual Basic .NET.

Microsoft .NET, which I refer to as just .NET, is a platform for developing “managed” software. The word managed is key here—a concept setting the .NET platform apart from many other development environments. I'll explain what the word managed means and why it is an integral capability of the .NET platform.

The .NET Framework is an application development platform that provides services for building, deploying, and running desktop, web, and phone applications and web services.

.NET Core is a general purpose development platform maintained by Microsoft and the .NET community on GitHub. It is cross-platform, supporting Windows, macOS and Linux, and can be used in device, cloud, and embedded/IoT scenarios.

The .NET Framework is a development platform for building apps for Windows, Windows Phone, Windows Server, and Microsoft Azure. It consists of the common language runtime (CLR) and the .NET Framework class library, which includes classes, interfaces, and value types that support an extensive range of technologies.

Introducing ASP.NET 5 is an open source web framework for building modern web applications that can be developed and run on Windows, Linux and the Mac. It includes the MVC 6 framework, which now combines the features of MVC and Web API into a single web programming framework.