The .Net Framework is basically a software framework designed by Microsoft which runs primarily on Microsoft Windows. This is a big collection of different libraries which is named as Framework Class Library (FCL). These libraries are further the collection of classes and functions.
Framework Class Libraries provides important features like:
User interface, data access, database connectivity, cryptography, web application development, numeric algorithms, and network communications.
.Net Framework libraries are designed to extend the CLR(Common Language Runtime) and to make the software and system development a lot easier.
CLR is a runtime engine which has a list of useful features like Just-In-Time compiling, Garbage Collection, etc.
Here is a pictorial representation of .Net Framework.