The robots compete in a variety of categories, with the small ones (that look more like hockey pucks than bots) limited to a size of 180mm in diameter and no higher than 15cm, and there's also a medium-sized league, and a four-legged league of Aibo-style bots that operate fully autonomously with no human interaction or external control. They are arranged in teams of four, playing on a 6m x 4m field.
These humanoid bots are divided into two classes, kid size (30-60cm in height) and teen size 80-130cm), and they have human-like senses and can run after the ball, kick it, and make their own decisions. So far, they look pretty lame, and remind us of small children just learning how to play soccer with their parents setting up all the shots. But if you consider what they're actually doing, it's downright remarkable. Check out the little goalie making a save.
RoboCup World Championship [RoboCup 2007 Atlanta]