The purpose of the game is to move the ball towards, and ultimately into, the opposition's end zone.
This is achieved by either running with the ball until tackled, or throwing the ball downfield to a team-mate.
DOWNS Downs are the most fundamental, and confusing, part of the NFL rulebook.
The attacking team, or offence, needs to move the ball forward in chunks of at least 10 yards, which is why the pitch has yardage markings.
They have four chances, or downs, to gain those 10 yards.
When the ball has advanced that far anotherfirst down is earned, with four more chances to go a further 10 yards.
If the offensive team fails to move 10 yards within four downs, possession is surrendered, although the ball is usually kicked, or punted, to the defending team on fourth down.
While most scoring comes from near the defending team's end zone, a touchdown can be scored from anywhere on the field, and on any down.
PLAYS These are the complicated movements involving all 11 players teams use to move the ball downfield.
Plays are called by the head coach or quarterback, who is the attacking lynchpin of any team.
While the term usually refers to attacking teams, defending teams also use set plays to stop their opponents' forward movement.
The offensive team has 30 seconds to get the ball in play or kick a field goal.
SCORING Touchdown (six points) A touchdown is scored when a team crosses the opposition's goal line with the ball, or catches or collects the ball in the end zone.
Field goal (three points) These are usually attempted on fourth down if the kicker is close enough to the end zone to kick the ball through the posts, or uprights.
Extra point (one or two points) A point is earned by kicking the ball through the uprights after a touchdown (similar to a rugby conversion). Two points are earned by taking the ball into the end zone again.
Safety (two points) Awarded to the defensive team when a member of the offensive team is tackled with the ball in his own end zone.
POSITIONS Although there are only 11 players from each side on the field at any one time, an American football team is made up of 45 players, ranging in size, speed and their roles within the team.
The offence A team's attacking players who attempt to move the ball forward and score touchdowns.
The key man is the quarterback.
On the quarterback's signal, the centre snaps the ball back between his legs to the QB (similar to rugby league's play-the-ball).
A typical offensive line-up
The quarterback can then either run with the ball, hand it to a running back, or pass it downfield to one of his receivers.
1 & 11 Wide receivers 2 & 6 Tackles 3 & 5 Guards 4 Centre 7 Tight end 8 Quarterback 9 Fullback 10 Running back
The plays used by the quarterback can either be pre-arranged or improvised to take advantage of the game's changing shape.
The defence The role of the defence is to stop the other team from scoring by tackling the ball-carrier, intercepting passes or causing fumbles.
A '3-4' defensive set-up
The defence is made up of big, powerful players trying to stop the other team from running the ball, and fast, athletic players trying to prevent the quarterback's passes from reaching his receivers.
The defence's power players also attempt to tackle, or sack, the quarterback before he has thrown the ball, while any defender can cause a fumble by knocking the ball from a ball-carrier's grasp.
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