Anytime you see the word framework in software development, it means it's a collection of things to make your job easier. The simplest answer I could give you is that the .NET framework is a collection of tools to make software development better, easier, and faster. It comes with an array of languages (C#, VB.NET, J#, to name a few), compilers, pre-made code and utilities in classes, controls, assemblies, etc. With these tools, if you wanted to create a server, for example, there would be already code available in that's part of the framework to make your job easier. Thus the word "framework" because you've used all these tools to build something.

Best place to start would be http://asp.net.