The National Football League (NFL) is the king of all football leagues. The best NCAA players are drafted to NFL teams every April. The biggest game of the year is the Super Bowl, which is typically held in early February. The NFL preseason schedule begins in late July/August and the regular season starts in September.
The Canadian Football League (CFL) is the top football league in Canada. The Canadian style of football is slightly different from the American-counterpart. The size of the football field is bigger and the rules are a little different. Most notably, the 3 downs to make a first down and the number of players allowed on the field per team is 12. Many players on CFL rosters derive from the NCAA.
The Arena Football League (AFL) is action-packed football played inside an indoor arena. The field size is much smaller and only 7 players per team are allowed on the field.
NCAA Football is the college/university organization for American football. There are over 100 teams in the top division of NCAA and there are 3 levels of divisions. Several NCAA football games are played in 100K+ in attendance.