Football

Match set-up
Football is a timed sport. The team with the most points at the end of the time period wins the game. The game is divided up into 4 periods or quarters with a long "half time" between the second and third quarter. Time is counted while plays are running and sometimes between plays. To keep the game going at a good pace the offense has a limited time between plays.

Football Players
The rules in football allow each team to have 11 players on the field at a time. Teams may substitute players between plays with no restrictions. Each team must start a play on their side of the ball.
The defensive players may take any position they want and can move about their side of the football prior to the play without restriction. There are certain defensive positions that have become common over time, there are no specific rules defining defensive positions or roles.
The offensive players, however, have several rules that define their position and what role they may take in the offense. Seven offensive players must be lined up on the line of scrimmage. The other four players must be lined up at least one yard behind the line of scrimmage. All of the offensive football players must be set, or still, prior to the play beginning with the exception of one of the 4 backs which may be moving parallel or away from the line of scrimmage. Further rules say that only the 4 backs and the players at each end of the line of scrimmage may catch a pass or run the football.

Now Play Football
The team with the possession of the football is called the offense. The offense tries to advance the football on plays. The defense tries to prevent the offense from scoring or advancing the football. The down system: The offense must advance the ball at least 10 yards every four plays or downs. Each time the offense is successful in advancing the ball 10 yards, they get four more downs or what is called a "first down". In order to keep the other team from getting good field position the offense can punt the ball to the other team intentionally. Offensive plays on downs start with a snap. This is when the center passes the football between their legs to one of the offensive backs. The ball is advanced either by running with the football or passing the football. The football play is over when 1) the player with the football is tackled or goes out of bounds 2) an incomplete pass 3) there is a score.