No tackling allowed
The most notable difference between flag football and tackle football is the absence of tackling in the flag game. All players wear flags that hang down at their sides, usually from belts worn around the waist. A ball carrier is "tackled" or down at the spot at which a defender removes one of the flags. If a ball carrier reaches the goal line with both flags intact, he scores a touchdown.
Not necessarily a non-contact sport
Flag football exists in many forms. Rules exist for everything from four-man teams to nine-man teams. Additionally, some flag football leagues allow contact, or blocking, while others do not. Full-contact flag football rules allow blocking, but restricts the type of blocking permitted to help avoid injury. Non-contact flag football prohibits blocking of any kind.
Catch that pass, lineman
A further distinction between flag football rules is the eligibility of linemen to catch passes. Unlike tackle football, in which the linemen are never eligible to catch a pass, some contact flag football leagues do allow linemen to catch passes as any receiver would in a tackle football game.
The United States Flag Football Association, which utilizes eight-man teams, requires fields to be 80 yards long with 10-yard end zones--20 yards shorter than a typical tackle football field. Rather than markings every 10 yards on the field, a flag football field is marked every 20 yards, reflecting the rule that teams must advance the ball 20 yards in four downs, rather than the 10-yard rule in tackle football. Other flag football leagues may use smaller fields for games with fewer players.
Watch the flag, not the ball
Scoring is essentially the same in flag football as it is in tackle football, although some leagues play with one exception--a pass for conversion after a touchdown is worth one point; a run for conversion is worth two points. However, a touchdown or conversion is not awarded when the ball crosses the goal line in flag football as it is in tackle football. Those points are awarded only after the ball carrier's flag has broken the plane of the goal.
Flag football enforces additional rules
Flag football has adopted many of the same penalties that tackle football uses. Because of the nature of the flag game, though, additional penalties covering acts such as elbowing, kneeing, protecting flags, illegal removal of flags and running into an opponent are enforced.