You've probably heard of Reflector, which lets you decompile .NET Assemblies and see how they work.  It's a very nice tool, for what it does.  However, as has been noted by others, if you're decompiling a rather large assembly, figuring out how everything links together can be difficult.  This is where something like Sacha Barber's 100% Reflective Class Diagram Creation Tool.  As you can guess from the name, it uses reflection to accomplish it's task, much like Reflector does.

Features:

• Detection of NON-CLR type being requested by user
• Customization of what is shown on the class diagram
  • Show interfaces [Yes / No]
  • Show constructor parameters [Yes / No]
  • Show field types [Yes / No]
  • Show method arguments [Yes / No]
  • Show method return values [Yes / No]
  • Show property types [Yes / No]
  • Show events [Yes / No]
  • Show enumerations
  • Show delegates [Yes / No]
  • Number of columns to use for the generated diagram (Number between 1-5)
  • Class background start color
  • Class background end color
  • Class border color
  • Accessibility modifier selection (Public only / Public and Static / All)
• Automatically drawn class association lines
• Automatically drawn generalization (inheritance) arrows
• Expand individual sections of a class (Constructors / Fields / Properties / Methods / Events may all be collapsed / expanded individually)
• Expand entire class
• Class representation as similar to Visual Studio 2005 look and feel as possible
• Allow saving of diagram to the following image formats (Bmp, Emf, Exif, Gif, Jpeg, Png)

Check it out.