Hi Friends,
.NET is a Framework by Microsoft that can be utilized for developing Web sites, but just as well for Windows programs, Windows Services, Web services, mobile apps and, with .NET Core and Mono, develop applications for Linux etc. The .NET Framework is just a framework, the underlying programming is done in a language such as C#, C++, F#, VB. NET etc.

Web development is as it sounds, developing stuff for the web alone.