There are a number of levels of American Football in the United States of America:
National Football League is the Major League in American Football.
US College Football is almost as popular as the NFL with the National Collegiate Athletic Association (NCAA) being the highest levels (Division I Football Bowl Series, Division I Football Championship Series, Division I AA, Division II, Division III) and below that Junior Colleges (e.g. NAIA)
Rival US Pro Football Leagues Almost every year since the birth of the NFL in 1920, there has been a Rival Major League or Development League (AAA Minor League).
Semi-Pro State Leagues form a level below Development Leagues, with most Leagues formed roughly along State lines.
American Football World Leagues Leagues around the World primarily featuring American players rather than Nationals.
There are other versions of American Football which form levels alongside or below these levels:
Reduced Numbers Leagues:
Arena Football an 8-a-side Indoor version of the game. Played in smaller localities (Pro or Semi-Pro)
6-Man Leagues: Popular in Texas.
8-a-side Leagues: Played in Smaller Localities, mostly for smaller High Schools.
9-a-side Leagues: Similar to 8-a-side Leagues.
Flag Football: the original Unkitted Football – played with a flag attached to the waist which is pulled off in order to tackle the ball carrier.
Lingerie / Legends Football Leagues: Unkitted Football featuring Scantily-Clad Women.
American 7s: Unkitted Contact Football, first codified in New Jersey 2006-2011. Bringing American Football back to its Rugby Roots.
Football Games e.g. Board Games.
E-Football: e.g. Fantasy Football, Console Games, Elimination Games.