In college sports, there are three main divisions of competition: Divisions I, II, and III. Of these, Division I is the most recognizable. For the sport of football, Division I is also split into two subdivisions: Division I-A and Division I-AA.
Division I-A, also known as the Football Bowl Subdivision (or FBS) has 129 programs and is easily the most recognizable of the divisions. These schools compete in individual bowl games at the end of the regular season, including the College Football Playoff.
Division I-AA, or the Football Championship Subdivision, is comprised of 125 programs that compete in a tournament-style championship bracket at the end of their regular season.
There are games between teams across subdivisions, but these teams are not in the same conferences and don't compete against one another in the postseason.
All these divisions ensure that the level of competition is somewhat level. FBS teams are allowed to offer more scholarships, generally have bigger stadiums and better facilities, and generate more revenue. They also must meet baseline attendance requirements.