Any application that runs in .Net is compiled to MSIL and stored as .dll files. Sometimes it would be handy to be able to de-compile the code, and examine the source functionality. This is especially true when you've lost the source code to an application you've created!
I had a situation not too long ago where I built a small ASP.net application to transmit and receive XML. However, after I deployed it live, my computer crashed and I hadn't backed up the source files. So I searched the web high and low, and was able to find a little tool called a reflector, which enables you to decompile the .dll, and see your source code that goes into it.
You can download the decompiling tool here for free!