It generates a file which contains the contents of the processors registers and flags. It also provides a dump of the stack so that you can see how the program got to where it is. It also contains information about the core system (version, sp state etc).
It's one of windows most utterly useless features as it sends the information about app crashes to microsoft, when it should send it to the software vendor - I think they've fixed this in vista though.
If you're entirely paranoid then it's more than possible that private information could be in the stack.