It took a while but here's a summary of the game. As I stated before, the rules are highly complex so I've had to leave plenty of stuff out, but unless I've missed anything glaringly obvious (and I'm sure fans will point it out to me if I have) then this should at least give you enough info to watch a game and have a basic understanding of what is happening.
Summary of American Football
American Football is played on a field 120 yards long and 53.33 yards wide. The 10 yards at both ends of the field are known as "end zones". Goalposts are located at the back of each end zone. The crossbar of the goalposts is 10 feet above the ground, the vertical uprights are 18 feet 6 inches apart, and extend up 35 feet.
Each team is made up of three units: offensive, defensive, and special teams. Each of these units contains 11 players (players can be on more than one unit), and teams put one unit on the field at a time, depending on the type of situation. Offensive units are used by the team with possession of the ball, and they attempt to get the ball down the field to score. Defensive units are used by the team who does not have possession of the ball, and attempt to stop the other team from scoring. Special teams units are used when a team is kicking the ball.
Scoring
There are a number of ways to score in American Football:
Touchdown (6 points) - A touchdown is scored whenever a player has possession of the ball within the opponent's end zone. If the player had possession of the ball outside of the end zone, all they need to do is make any part of the ball cross the line at the front of the end zone (called the plane) to score. If a player does not have possession of the ball, but catches it whilst they are in the end zone and have both feet in bounds, they also score.
Field Goal (3 points) - A field goal is scored whenever a team's kicker kicks the ball through the goalposts of the opponent. The ball has to go above the crossbar and between the uprights.
Safety (2 points) - A safety is one of the rarest ways to score. If a player happens to possess the ball in their own end zone and is either tackled down or steps out of bounds, the other team scores a safety.
Point(s) After Touchdown (PAT) (1 or 2 points) - After a touchdown is scored, the scoring team can either attempt to kick the ball through the uprights (like scoring a field goal) for a single point, or try to take the ball into the end zone again (like a touchdown) for 2 points. Unless the team is losing badly, most teams will go for 1 point rather than 2 points, as it has a higher success rate.
There are a few other ways to score safeties, but they are so rare it's not worth mentioning them in this summary.
Kickoffs
At the beginning of each half, there is a kickoff. A kickoff also happens when a team scores (with the scoring team kicking the ball to their opponent). The ball is placed at the 35 yard line of the kicking team, and is kicked to the other team (called the receiving team), who then gains possession of the ball. Usually the kicking team tries to get the ball as far away from their own end zone as possible.
The team receiving the ball can catch the ball and attempt to run with it until they are either tackled down, or the person with the ball runs into the end zone and scores a touchdown (which is unlikely but it happens). Alternatively, a player on the receiving team can signal for a fair catch whilst the ball is in the air, catch it, and the ball will be placed at the point of the catch.If the kicking team kicks the ball into the receiving team's end zone and it is downed, or the ball goes beyond the end zone, it is said to be a touchback, and the receiving team starts with the ball at their own 20 yard line.
Usually the kicking team will either attempt to kick a touchback, or try to tackle the receiving team's ball carrier before they reach the 20 yard line.
Plays / Downs
When a team gains possession of the ball they attempt to "drive" the ball downfield to score. The ball is placed at the line of scrimmage, and the offensive unit are given 4 attempts (called downs) to get the ball to the first down line, which is 10 yards away from the line of scrimmage. If they succeed, they are given a new set of 4 downs, and have to gain at least 10 yards again to continue the drive.
On the 4th down, if the offensive unit is in field goal range (i.e. they think it is likely that they will score a field goal) then they will bring out their kicking unit and attempt the field goal. If they are out of field goal range and don't need the points, they will punt the ball (drop kick it) to the other team, who will then gain possession. If they are out of field goal range but do need the points, they may attempt to gain the needed yards on 4th down.
The "1st and 10" system is used on TV and in stadiums to let spectators know what the current situation is on the field. "1st and 10" means it is the 1st down, and the offense needs to move the ball 10 yards to gain a new set of downs. Likewise, "2nd and 3" means it is 2nd down, and the offense needs to move the ball 3 yards. If the first down line is at the end zone, it is called "1st and Goal".
The ball can be moved down the field by either rushing or passing. When rushing, a player on the offense takes the ball and runs with it, attempting to gain the needed yardage until the ball is considered to be "dead". When passing, a player (usually the Quarterback) takes the ball and throws it forward to another player, who then has to catch the ball and attempts to run with it until the ball is considered to be "dead". One forward pass is allowed every down, and the pass must begin (i.e. the ball must leave the player's hand) whilst they are behind the line of scrimmage. Teams are allowed unlimited "lateral" passes (passes which are thrown backwards or laterally) in a play, but these are very rare, and are usually only used in a last ditch attempt to get points before the end of a game.
If a ball becomes "dead" behind the line of scrimmage, the offense will usually lose yards. For instance, if it is 1st and 10, and the ball is handed to a running back 5 yards behind the line of scrimmage and he is immediately tackled, the offense will lose 5 yards and it will be 2nd and 15. If the player gains yards but is then pushed back and tackled, they will be given the yards gained as "forward progress". For instance, if the running back gains 2 yards but is pushed back 5 and tackled, it will be 2nd and 8 rather than 2nd and 13.
A play ends when the ball becomes "dead", which usually happens when the ballcarrier is tackled (a defensive player touches the ball carrier and either their knee or elbow touches the ground) or the ballcarrier steps out of bounds (with any part of either foot). The ball also becomes "dead" if a pass ends up with the ball hitting the ground before a player gains possession of it (for instance, if it is overthrown, or bounces off a receiver, or the receiver catches it but it is immediately knocked out).
Turnovers
Turnovers happen when a team is forced to give up possession of the ball without scoring. There are three ways in which turnovers can happen:
Turnover on Downs - If the offensive unit attempts to move the ball on 4th down and fails to gain the necessary yards, it is called a turnover on downs, and the ball is given to the opposing team at the point where the ball was last brought down.
Fumble - If an offensive player has possession of the ball but drops it before they are tackled down or step out of bounds, it is called a fumble. The ball can be recovered by either the offense (who will retain possession of the ball), or by the defense (who will gain possession of the ball).
Interception - If the offense attempts to pass the ball but it is caught by a member of the defense, it is called an interception, and the defense gains possession of the ball.
Penalties
There are numerous ways to commit penalties, and they can be committed by either team. It's not worth explaining them all here, but penalties committed by the offense result in a loss of yards for the offense, whilst penalties committed by the defense result in a gain of yards for the offense (and can also result in automatic 1st downs).
Clock Management
An important aspect of the game is clock management. The entire game lasts 60 minutes, however this is split into four 15 minute "quarters". At the end of each quarter, the clock resets to 15:00 the teams change ends. Whilst the teams change ends, there is a short break (sometimes a few minutes). Half-time is a 12 minute (usually) break between the 2nd and 3rd quarters. Kickoffs occur at the beginning of the 1st and 3rd quarters with alternate teams (i.e. the team that kicked off in the 1st quarter will not kick off in the 3rd).
On kickoff, the clock starts running when the receiving team catches the ball, and stops when the ball carrier is tackled down, goes out of bounds, or scores a touchdown. If a touchback occurs, the clock doesn't start at all. The clock starts again when the offense "snaps" the ball (the action of the center picking up the ball from the line of scrimmage and giving it to the Quarterback). Once the clock starts, it will only stop if any of the following occurs:
- The ball carrier goes out of bounds within the last two minutes of the first half or the last five minutes of the second half.
- A forward pass is incomplete.
- Either team calls for a time-out (each team gets 3 time-outs per half) or an official calls for a time-out (usually because of an injury or a penalty).
- Either team scores.
- A turnover occurs.
If the clock runs down to 0:00 in any quarter and the ball is not dead, play continues until the ball becomes dead. This has led to several interesting plays where the offense has managed to win the game when there was technically no time left on the clock.