Although brian's link will get you to a lot of information about the framework, perhaps this simple explanation will give you a good overview:
The .NET Framework is a run-time environment. In the past you may have used VB6, VBScript, Java or something similar. In order to write a program in one of those languages, you needed their respective run-time libraries so the processor knows what to do with the commands you provide in the run-time specific language. .NET is similar but brings a whole knew aspect to development (particularly for those of us coming from a VBx background). Major advantages to the .NET framework are:
- Managed code
- CLR or Common Language Runtime
- Universal Namespace
All languages that can be used to write a .NET application must be CLR compliant. All the languages supported can utilize the same .NET namespace classes. This means you can write in one language and your neighbor can write in another. But the pieces of the program that you each write can work together because they both speak the same base language of the .NET runtime.
When a .NET application is compiled, regardless of the language it was written in, the code is distilled down to Microsoft Intermediate Language (MSIL) which is like the universal .NET language. The resulting code lands in a .NET assembly, in the form of an EXE or a DLL file. When you actually run this code, the MSIL is pumped thru the Just-In-Time (JIT) runtime compiler and then executed. The eventual goal is that you can have JIT runtimes that can run on different platforms, but the source .NET application code (the assemblies) are the same. The .NET runtime (JIT) knows how to change the MSIL code into instructions that are specific to the OS platform the runtime is living on.
The .NET framework contains an enormous amount of functionality and capability, and will contain significantly more in the next release (2.0). You can use it to build windows applications of all flavors, web applications and services (my areas of expertise) and more. My best suggestion is to just read about whatever you can to get a better understanding of it all. It took some time to understand it, but eventually I saw the light and realized how great it is and will be.
Work smarter, not harder