Flag football is a version of American football where the basic rules of the game are similar to those of the mainstream game (often called « tackle football » for contrast), but instead of tackling players to the ground, the defensive team must remove a flag or flag belt from the ball carrier (« deflagging ») to end a down. The specific rules of flag football vary widely by league, though all share in common their replication of the rules of traditional American football with tackling replaced by flag-pulling. Traditional American football rules are often omitted or simplified to reflect the more recreational nature of the game, desire to avoid physical contact and injury, and the generally smaller number of participating players per side.

For even more information: http://ifaf.org/pdf/flag/ifaf-flag-rules.pdf