One 11-man team has possession of the football. It is called the offense and it tries to advance the ball down the field-by running with the ball or throwing it - and score points by crossing the goal line and getting into an area called the end zone. 

The other team is called the defense. It tries to stop the offensive team and make it give up possession of the ball. If the team with the ball does score or is forced to give up possession, the offensive and defensive teams switch roles (the offensive team goes on defense and the defensive team goes on offense). And so on, back and forth, until all four quarters of the game have been played.

In order to make it easier to coordinate the information in this digest, the topics discussed generally follow the order of the rule book.

THE FIELD

The field measures 100 yards long and 53 yards wide. Little white markings on the field called yard markers help the players, officials, and the fans keep track of the ball. Probably the most important part of the field is the end zone. It's an additional 10 yards on each end of the field. This is where the points add up! When the offense - the team with possession of the ball-gets the ball into the opponent's end zone, they score points.

Games are divided into four 15-minute quarters, separated by a 12-minute break at halftime. There are also 2-minute breaks at the end of the first and third quarters as teams change ends of the field after every 15 minutes of play. At the end of the first and third quarters, the team with the ball retains possession heading into the following quarter. That is not the case before halftime. The second half starts with a kickoff in the same way as the game began in the first quarter.

Each offensive team has 40 seconds from the end of a given play until they must snap the ball for the start of the next play, otherwise they will be penalized.

The clock stops at the end of incomplete passing plays, when a player goes out of bounds, or when a penalty is called. The clock starts again when the ball is re-spotted by an official.

If a game is tied at the end of regulation, a 15-minute overtime period will be played. In the NFL, this is sudden death and the first team to score wins. Possession is determined before the period begins by a coin toss.

Each team has 3 separate units: the offense (see section below), those players who are on the field when the team has possession of the ball; the defense (see section below), players who line up to stop the other team's offense; and special teams that only come in on kicking situations (punts, field goals, and kickoffs). Only 11 players are on the field from one team at any one time.

A game starts with the kickoff. The ball is placed on a kicking tee at the defense's 30-yard line, and a special kicker (a "placekicker") kicks the ball to the offense A kick return man from the offense will try to catch the ball and advance it by running. Where he is stopped is the point from which the offense will begin its drive, or series of offensive plays. When a kickoff is caught in the offense's own end zone, the kick returner can either run the ball out of the end zone, or kneel in the end zone to signal a touchback - a sign to stop the play. The ball is then placed on the 20-yard line, where the offense begins play.

All progress in a football game is measured in yards. The offensive team tries to get as much "yardage" as it can to try and move closer to the opponent's end zone. Each time the offense gets the ball, it has four downs, or chances, in which to gain 10 yards. If the offensive team successfully moves the ball 10 or more yards, it earns a first down, and another set of four downs. If the offense fails to gain 10 yards, it loses possession of the ball. The defense tries to prevent the offense not only from scoring, but also from gaining the 10 yards needed for a first down. If the offense reaches fourth down, it usually punts the ball (kicks it away). This forces the other team to begin its drive further down the field.

A play begins with the snap. At the line of scrimmage (the position on the field where the play begins), the quarterback loudly calls out a play in code and the player in front of him, the center, passes, or snaps the ball under his legs to the quarterback. From there, the quarterback can either throw the ball, hand it off, or run with it.

There are two main ways for the offense to advance the ball. The first is called a run. This occurs when the quarterback hands the ball off to a running back, who then tries to gain as many yards as possible by eluding defensive players. The quarterback is also allowed to run with the ball.

The other alternative to running the ball is to throw it. Or as they say in football, pass it! Usually, the quarterback does the passing, though there are times when another player may pass the ball to confuse the defense. Actually, anyone on the offensive team is allowed to pass the ball as long as the pass is thrown from behind the line of scrimmage. A pass is complete if the ball is caught by another offensive player, usually the "wide receiver" or "tight end." If the ball hits the ground before someone catches it, it is called an incomplete pass.

The defense prevents the offense from advancing the ball by bringing the ball carrier to the ground. A player is tackled when one or both of his knees touch the ground. The play is then over. A play also ends when a player runs out of bounds.

The object of the game is to score the most points. There are four ways to score points in football.

A touchdown is the biggest single score in a football game. It is worth six points, and it allows the scoring team an opportunity to attempt to get an extra point. To score a touchdown, the ball must be carried across the goal line into the end zone, caught in the end zone, or a fumble recovered in the end zone, or an untouched kickoff recovered in the end zone by the kicking team.

EXTRA POINT and the TWO-POINT CONVERSION = 1 or 2 POINTS                

Immediately following a touchdown, the ball is placed at the opponent's two-yard line, where the offense has two options. Usually the offense will kick an extra point, also called the point after touchdown, conversion, or PAT. If the offense successfully kicks the ball through the goal posts, it earns one point. The offense can also score two points by running or throwing the ball into the end zone in the same manner as you would score a touchdown. Since going for two points is more difficult than kicking an extra point, the offense generally chooses to kick the extra point.

If the offense cannot score a touchdown, it may try to kick a field goal. Field goals are worth three points and often are the deciding plays in the last seconds of close games. They can be attempted from anywhere on the field on any down, but generally are kicked from inside the defense's 45-yard line on fourth down. For a field goal to be "good", the placekicker (or field goal kicker) must kick the ball through the goal-post uprights and over the crossbar. The defense tries to block the kick and stop the ball from reaching the goal post.

The safety is worth two points. A safety occurs when the offensive ball carrier is tackled behind his own goal line.

While trying to advance the football to the end zone, the offense may accidentally turn the ball over to the defense in one of two ways:

When the ball carrier or passer drops the ball, that's a fumble. Any player on the field can recover the ball by diving on it or he can run with it. The team that recovers a fumble either gets-or retains-possession of the ball.

An aggressive defense can regain possession of the ball by catching (intercepting) passes meant for players on the other team. Both fumble recoveries and interceptions can be run back into the end zone for touchdowns.

THE TWO SIDES OF THE BALL 

Whichever team has possession of the ball is the offense. While only the quarterback, the wide receivers and tight ends, and the running backs can legally handle the ball, it is the quarterback who is the leader of the team and the playmaker. In fact, he's a man of many talents - he not only throws the ball, he outlines each play to his team.

The quarterback ("QB") passes or hands off the ball.

The center snaps the ball to the QB and blocks the defense.

2 guards and 2 tackles keep the defense at bay.

2/4 wide receivers catch the ball thrown by the QB.

1 or 2 running backs take the ball and run with it.

1 or 2 tight ends block the defense and can also catches passes.

The job of the defense is to stop the offense. The 11 men on the defensive team all work together to keep the offense from advancing toward the defense's end zone.

Linebackers defend against the pass, and push forward to stop the run or tackle the QB.

The defensive line (ends and tackles) battles head-to-head against the offensive line.

Cornerbacks and safeties defend against the pass from the QB to the wide receiver and help to stop the run.