American football is played on a field 100 yards long between two teams of 11 players. The goal is to score more points than the other team by carrying the ball into the end zone (6 points) or by kicking field goals (3), as well as other ways to score.
In college and the National Football League, each game lasts 60 minutes, divided into four quarters. The game begins with a kickoff (the receiving team usually is determined by coin toss), and each time a team scores, it kicks off to the opponent.
Each team takes turns trying to move the ball into or close to the opponent's end zone by either running the ball or throwing it. Getting the ball in the end zone results in a touchdown. Teams also can score by kicking field goals -- booting the ball through the upright goal post from some distance outside the end zone.
The offense must move at least 10 yards every four plays to earn a first down and maintain possession. If a team has not gained a first down by the fourth play, it must punt, kick a field goal or run a play to try to gain the needed yards for the first down. The distance required for the first down or a field goal generally determines the strategy.
Each play begins with a center snap in which one of the middle five players hikes the ball to the quarterback, who then hands the ball to a running back or throws it to an eligible receiver. The quarterback also can run with the ball.
The defense tries to tackle the ball carrier or prevent passes from being completed, either by knocking them down or intercepting them. The defense can score, as well, either via takeaways (interceptions and fumbles) that turn into touchdowns or by safety (tackling the offense in its own end zone, good for two points for the defense's team).
The team with the most points at the end of 60 minutes (or overtime) wins.