Object of the game
The object of the game is to advance the ball to the opponent's end zone and thus score points. The team with the most points when time has expired wins.

Methods of scoring
Points can be scored in the following ways:

1.) A field goal, worth 3 points, is scored by kicking the ball between the uprights of the goal posts. In the NFL Europe league, Field Goals over 50 yards attract 4 points.

2.) A touchdown, worth 6 points, is scored when the ball is advanced into the opponent's end zone. Immediately after a touchdown, the scoring team may run a single play called a try (more commonly called an extra point or conversion attempt) from just outside the end zone. They can either attempt to kick the ball through the uprights (worth one point, for a total of seven) or advance the ball into the end zone (worth 2, for a total of eight) on this play.

A safety is a rare event occurring when a player causes the ball to become dead in his own end zone. When this occurs, the opposing team scores 2 points and gains posession. Typically this occurs when a ballcarrier is tackled in their own endzone, or fumbles the ball out of their own endzone.

Duration of game
Collegiate and professional football games are 60 minutes long, divided into four quarters of 15 minutes each. Separating the second and third quarters is a halftime. If a game is tied at the end of four quarters, overtime is played. Professional overtime is played in 15-minute "sudden death" periods, meaning that the team that scores first, by any means, wins. In college football, a shootout style overtime system ensures that each team has equal opportunity to score. Some High School games play 12-minute quarters instead of 15.

The Kickoff
A kickoff starts each half, and also restarts play following a field goal or touchdown. At the beginning of a half, the kicking team is determined by coin toss. After a team scores a field goal or touchdown, it kicks off the ball to its opponent. The ball is placed on a tee and kicked off at the kicking team's own 30-yard line in the NFL, or its own 35-yard line in college football. The ball is usually kicked as far as possible down the field, but sometimes a team will attempt to recover its own short kick, in a play known as an onside kick. The receiving team may catch and attempt to advance the ball at any time after the kick, but the kicking team may not touch the ball until it has traveled at least 10 yards.

Following a safety, a free kick similar to a kickoff is used to restart the game. The team that was scored upon kicks the ball from its own 20-yard line. In this case, a tee cannot be used, so the ball is usually punted.

Challenging plays
In the NFL, a team's coach may "challenge" a play if he thinks an official's ruling is incorrect. He does so by tossing a red flag (like the yellow flags used by officials to denote penalties) onto the field. The official that made the call then has 90 seconds to review multiple videotaped angles of the play, and then he either overturns or upholds the ruling. If the ruling is upheld, the team that challenged the play is charged with a time out. A team may usually challenge only two plays per game. However, if a team is successful on two challenges in a game, they will receive a third challenge. A play can only be challenged before the next one has started.

The officials may decide to review a play themselves if they do not all agree on the call. No team is charged with a challenge in this event. Only officials can review plays in the last two minutes ("two-minute warning") of each half or in overtime periods. This system was introduced in 1999.

Plays of the game
A game consists of many individual plays. The vast majority of these are scrimmage plays. Each play from scrimmage is one of a series of downs given to the team with possession. These two concepts, the concept of scrimmage, and the concept of downs, are fundamental to American football, and are what distinguish it, as well as Canadian football, from most other forms of football. However, rugby league does have a similar system where each side is allowed to be tackled six times while in possession before surrendering possession (see the entry for rugby league for an explanation of the play-the-ball and the limited tackles rule).

The team with possession of the ball is called the offensive team, and the other team the defensive team. A set of downs begins with a first down, which is given to a team either after it has just gained possession on the previous play, or it has gained the necessary yardage from a previous set of downs. On a first down, the offensive team is given four downs to gain 10 yards. This is commonly referred to as "first and ten", meaning that it is first down, and ten yards are needed to get another first down. The line a team must reach in order to gain a first down is technically called the line to gain or the necessary line, although it is commonly called the first down line. In the event a team gains a first down and the new line of scrimmage is within ten yars of their opponent's goal line, the goal line becomes line to gain. This is commonly known as "first and goal," signifying that no more first downs can be achieved, and it is necessary to score on the current set of downs. Failure to gain the necessary yardage on a set of downs results in a "turnover on downs."

Plays from Scrimmage
Each down is a play from scrimmage. Prior to each play from scrimmage, the two teams line up on opposite sides of a line of scrimmage, which is defined by the spot of the ball from the previous play. The spot is, in most cases, the yard line at which the ball became dead on the previous play, plus or minus any penalty yardage. A down, or play from scrimmage, begins with a snap and ends when the ball becomes dead for any reason. In a snap, the center either hands the ball between his legs to the quarterback, or tosses it backwards between his legs to the quarterback or sometimes another player, such as a punter or a holder for a field goal attempt. The ball may become dead, ending the down, because a player in possession is tackled, or because his forward progress is stopped, or because he goes out of bounds, or because a forward pass goes incomplete, or because a player makes a "fair catch" (see punts below). Each play from scrimmage can be either an attempt to advance the ball, a punt, or an attempt a field goal.

Advancing the ball
There are two methods of advancing the ball while still maintaining possession:

1.) Running the ball - The quarterback, who normally receives the snap, either hands the ball or throws a lateral pass to a running back, who then becomes the ball carrier. Most other players on the offense have blocking assignments, and attempt to prevent the defense from tackling the ball carrier. The quarterback may also run the ball himself.

2.) A forward pass - A forward pass may only be thrown on a play from scrimmage, and only from behind the line of scrimmage. It must be thrown to an eligible receiver (any player who is not an interior lineman). A completed pass is one caught by a member of the offense, although if the first player to touch the ball is not an eligible receiver, a penalty results. The player may run with the ball after catching it. To be considered "in bounds" a receiver must have clear possession of the ball and place both feet (NFL) or one foot (college) in bounds prior to stepping out of bounds. An incomplete pass is any forward pass that either hits the ground or goes out of bounds, at which point the ball becomes dead, and is spotted at the preceding line of scrimmage for the following play. An interception a pass caught by the defense, which transfers possession to the defending team.

It is important for the offense to run a variety of running and passing plays in order to keep the defense uncertain of the next play.

Fourth down situations
If a team uses all four of its downs without gaining the yardage for a first down or a touchdown, and without kicking a field goal, possession shifts to the other team. This is called turning the ball over "on downs." Fourth down situations are therefore pivotal. The offense has the same three choices as on any other play from scrimmage: advance the ball, punt, or attempt a field goal — but the decision is often more difficult and important.

Offensive options on fourth down:

1.) "Go for it" - despite the risk involved, a team may always elect to "go for it" on fourth down by making one last attempt to reach the first down marker or the goal line, mounting a regular running or passing play to get there (just as they did on the first three downs). This is most common when, due to a team's success on the first three downs, the distance required for a first down is short; or when it is trailing late in the game by more than three points (the value of a field goal). The risk is significant: failing to make the next first down or score gives possession of the ball to the opposing team, usually with better field position than would have resulted from a kick. It's often wise to kick on fourth down.

2.) Punt - If the team thinks it is too far away to attempt a field goal, it may punt the ball to the other team in order to gain better field position.

3.) Attempt a field goal - Field goal attempts must be made with the ball on the ground (they cannot be punted), so a player called a holder holds the ball for a kicker. In times past, a kicker might have tried a "drop kick" — that is, dropping the ball and kicking it after it bounces off the ground — and if the kicker kicks it through the goalposts, it is a field goal. This is difficult to do, as the ball is in the shape of a prolate spheroid and its bounce is unpredictable. Nowadays, the only time you will see this is by a hurried kicker after a broken play. Failed field goal attempts, if they are short, can be returned by the opponent, but the ball usually goes past the end line and can't be returned. If the field goal attempt fails, the ball is spotted at the original line of scrimmage, and possession is given to the other team. (In the NFL, failed field goal attempts are spotted at the spot of the kick or the 20, whichever is farther from the goal line.) Field goals can also be attempted on other downs, but this is only seen in situations where a field goal will either win or tie the game and the distance to kick the field goal is well within range of the kicker.

A team will occasionally run a trick play on fourth down. They will line up in a punting or field goal formation, but will instead run the ball or pass it in an attempt to pick up a first down.

Specialized units and players
With its unlimited substitutions, American football is highly specialized, with most teams having three specialized units: an offensive unit, a defensive unit, and special teams.

There are many specialized players within each unit. Some players may only be used in certain situations. (for details see: offensive unit, defensive unit, special teams, linemen, defensive schemes.)

A list of player types and definitions can also be found in the Glossary of American football.