The .NET Framework is one of the programming models which supports the construction and operations of various applications for Windows, Windows Phone, Microsoft Azure, and XML internet services. It is created by Microsoft and operates on Microsoft Windows. The .NET Framework includes a large class library which is known as Framework category Library, and another one is called customary Language Runtime that provides the ability to understand the language across many programming languages.
This structure was designed to beat numerous issues of application advancement together with long development times, weakness to vary applications rapidly, cost of ownership of the software is quite high, and deployment was quite difficult.
Source: Web Dev