Every fall, there is an open registration for any interested teams for the Championships, which is usually limited to around 150-200 teams. Also, some teams, like the remaining six teams from the 1992 season and the Hall Of Fame teams, are automatically invited back every year.
The different divisions are all completely separate, and teams are equally divided among them as they count off by ascending team number. For most of the completion, the teams only play teams from their own division through qualification rounds, and then elimination rounds. Once there is the winning alliance from each of the four divisions, they move to a fifth playing field (Einstein) and duke it out there for the title of World Champion.
Edit: Wow, Michelle and Nawaid beat me to it by two minutes.
