Some time ago, I've written a post talking about Agile Software Development methodologies, where I mentioned Extreme Programming (aka XP).
I was thinking about writing a post on Extreme Programming, but I've just found a very good video of a lecture on the subject by Richard Buckland, that explains it the best way possible and with some humor. So here's the video:
This is a highly recommended video for those that want to learn about Extreme Programming and Unit Tests.
Video Link: http://youtube.com/watch?v=XP4o0ArkP4s