firstly, why you are not Gooooooled for your Question ?
.NET is hugh bunch of softwares that run on microsoft windows, includes a large library and supports several programming languages that allows you allow you to create, design and develope your own software
The .NET Framework's Base Class Library provides user interface, data access, database connectivity, cryptography, web application development, numeric algorithms, and network communications. Programmers produce software by combining their own source code with the .NET Framework and other libraries
check this link - Getting started with .NET