Microsoft .NET is the new development framework for writing stable, fast enterprise applications. Or anything else, really!
The .NET framework offers the run-time environment for any .NET compiled applications (build to something called IL - Intermediate Language - these require the .NET framework in order to run).
.NET introduces a new programming language - C# - as well as modifying Visual Basic and Visual C++ to support the new features of Managed Code. Possibly the greatest feature being... no more memory management issues with .NET's automatic Garbage Collection!
Have a browse through this section for info about the .NET framework in general, the programming language, and some examples of what you can do...