.NET is a software framework which is designed and developed by Microsoft. The first version of the .Net framework was 1.0 which came in the year 2002. In easy words, it is a virtual machine for compiling and executing programs written in different languages like C#, VB.Net etc.
It is used to develop Desktop applications, Web-based applications, and Web services. There is a variety of programming languages available on the .Net platform, C# and VB.Net being the most common ones. It is used to build applications for Windows, Mobile, Web, etc.