American football is a field game that is played between two teams using a ball whose shape might be described as a somewhat pointed oval. One team at a time has possession of the ball and tries to advance it down the field to score points, while the other team tries to stop the opponent and gain possession of the ball. The ball can be carried, passed or kicked, with special rules determining how those can be done and the outcomes of each action. Game action is divided into plays, which typically last only a few seconds. There are many variations of this sport, with the differences being factors such as the number of players on each team, how players can be stopped from advancing and the type and size of the ball and field.
In a standard game, each team uses 11 players at a time. A small percentage of organized leagues allow teams to use a different number of players on the field, such as seven, eight, nine or 12. Very small high schools, for example, might play eight-man football because each team has fewer players available. Informal games, such as those played between friends, might use any number of players per team.
A regulation football field is a total of 120 yards (109.7 m) long and 53.3 yards (48.7 m) wide, with horizontal lines painted every 5 yards (4.6 m) for the length of the field. The goal lines are 100 yards (91.4 m) apart, with a 10-yard (9.14 m) end zone behind each goal line. Leagues for youth teams often use smaller fields, and leagues in Canada typically use fields that are 10 yards (9.14 m) longer. One variation, indoor football, is played on artificial turf laid out on a hockey rink, so the field is much smaller and is surrounded by walls. Informal games are played on fields of varying sizes, usually without lines on the fields and with the goal lines represented by whatever objects might be available or handy — even trees or other plants where they already are growing.
In normal football games, players are allowed to make forceful contact with each other and tackle the player who has the ball, so they wear helmets and padding for protection. Some variations of the sport do not allow tackling or other such physical play. One variation is called touch football, in which a play ends as soon as the player who has the ball has been touched by an opponent — the touch can be with one hand or two, depending on the rules being used. Another variation is called flag football, in which players have one or more small strips of cloth — or "flags" — hanging from their waists, and opponents must grab the ballcarrier's flag to stop him or her.
A team's main objective is to score points by advancing the ball into the opponent's end zone for a touchdown. There are other ways to score as well, including tackling an opposing ballcarrier in the opposing team's end zone for a safety or kicking the ball through a pair goalposts, which are behind the end zones on standard football fields, for a field goal. After a touchdown, the scoring team is given another opportunity to score, called a try, and could run or pass the ball into the end zone or kick the ball through the goalposts. The various ways of scoring have specific names and are worth a certain number of points — touchdowns are six points, field goals are three points, safeties usually are two points and tries are either one point or two, depending on the method of scoring and the rules being followed. The team that scores the most points wins the game.
In most games, the action begins with a kickoff, when one team kicks the ball to the other and tries to tackle the player who catches it. The team with the ball then begins a possession and must advance the ball at least 10 yards (9.14 m) in four plays or less, or the other team will be given possession of the ball. A play begins with one player snapping the ball to another, usually by handing or passing it between the legs, and it usually ends when the player who has the ball is tackled, touched or has one of his or her flags taken; when the ball goes out of bounds; when points are scored; or when a ball that was passed forward touches the ground. On any play, the offense can choose to kick the ball in one of various ways, either to score points or to give the opponent possession of the ball as far away from the kicking team's end zone as possible.
Most games are divided into four quarters of equal length, such as 15 minutes, 12 minutes or eight minutes apiece. At the end of each quarter, the teams switch end zones. After the first two quarters, there is a halftime that typically lasts 12 or 15 minutes, during which players can rest and teams can discuss strategies. If the score is tied after all four quarters, there might be an overtime period of a specific length of time, or the teams might alternate possessions starting at a certain spot on the field until a winner is determined.
In most games, players are put in specific positions, with each position having certain responsibilities. On the offense, the team that has possession of the ball, the quarterback receives the snap and can run with the ball or pass it or hand it to a teammate. Running backs primarily run with the ball, receivers primarily catch passes, and linemen mostly block the opposing players. On defense, linemen try to push their way through the offensive line to get to the ballcarrier, linebackers mainly try to run to the ballcarrier as quickly as possible, and defensive backs try to prevent the offense's receivers from catching passes. A significant part of offensive and defensive strategy involves the types of players and formations used for each play as well as each player's specific assignment on the play.