Answer by Markus Jarderot for Ball to Ball Collision - Detection and Handling

I found an excellent page with information on collision detection and response in 2D.

http://www.metanetsoftware.com/technique.html (web.archive.org)

They try to explain how it's done from an academic point of view. They start with the simple object-to-object collision detection, and move on to collision response and how to scale it up.

Edit: Updated link