Simple question: What is a hash join?

Hi. I've been hearing about hash joins for quite a while now and I've always been kind of foggy on exactly what they are. As I understand it, the optimiser sometimes decides to do a hash join for a join between two tables, but I don't understand why. I'm guessing that if I knew what a hash join was, everything would be a lot clearer. Thanks, Steve Kain Peapod Inc.