Possession and Kickoff: NFL games begin with a coin toss at mid-field to determine possession of the ball. The visiting teams' captains are invited to call "heads" or "tails." (In neutral-site games, such as the Super Bowl, the referee designates which team calls the toss.) The team that wins the toss has several options for determining possession of the ball. They can elect to receive the ball at the start of play or to kick off and begin the game with their defensive unit on the field. (A kickoff, as the term implies, is when a team concedes possession of the ball to its opponents, kicking it downfield in hopes of pushing the other guys deep into their own territory.) Or they can choose which end zone to defend, thereby allowing the other team to determine initial possession.
When deciding possession, teams consider a range of factors. For example, a team with a particularly strong defense and a mediocre offense may find it effective to begin the game without the ball, on the theory that a quick and effective defensive series can yield enough momentum to jump-start a nothing-special offensive attack. In an open-ended stadium with a spiraling wind, it might be more of an advantage to begin the game with the wind at your back than to start out with possession of the ball.
At the beginning of the second half, the opening kickoff scenario is reversed, leaving open the possibility that a team could score on the last play of the first half and receive the ball on the ensuing kickoff to start the second half.
Since 1994, the ball has been placed on the kicking team's 30-yard line for each kickoff; prior to that, it had been placed on the 35-yard line, and prior to that on the 40-yard line. It was moved back over the years to compensate for the increased leg strength of professional place kickers, and to give the receiving team a little more room to mount an effective return.
Moving the Ball: Any player on the receiving team is eligible to catch, pick up, or otherwise gain possession of the ball on a kickoff, after which he can advance it towards the opponents' goal line until he is tackled or run out of bounds. A player is considered tackled, or "downed," when one knee touches the ground. A player is considered out of bounds when a finger, toe, elbow, or any other body part touches the white boundary line bordering the field.
Each team is given four downs (plays, attempts) to move the ball 10 yards down the field. So if you advance 3 or 4 yards on each play, you'd be in good shape over the course of the game. If a team achieves those 10 yards in four plays or less, it is awarded another four attempts -- "a new set of downs" in football parlance, -- to cover the next 10 yards. And so on. When there are no longer 10 yards left to gain, it usually means an offensive team has run across the opponents' goal line and scored a touchdown, accomplishing their primary objective -- unless of course a player has taken a few too many hits to the head and has run in the wrong direction, which, believe it or not, has been known to happen.
If after four downs an offensive team has been unable to gain those 10 yards or score a touchdown, it must give up possession of the ball to the defending team, whereupon the action on the field shifts direction. In this way, I guess, the game is a great model for teaching our preschool kids how to share -- each side gets a turn and no one gets to hog the ball without earning the right to hog it.
Once the ball shifts possession, the team that is now on offense must try to advance it toward their opponents' goal line; the team now on defense must try to keep them from doing so. It's in this back-and-forth that the game is played. (Talk about playing fair! Playing nice, however, doesn't seem to be in the game's lesson plan.)
Typically, a team will punt (kick) the ball on its fourth attempt if it hasn't gained the necessary first-down yardage, sending it downfield and pushing the opponent further away from its own goal line before giving up possession. However, if they're in field-goal range, they might go for the three points as a kind of consolation prize.
A play is considered stopped when the ballcarrier is tackled; when a ballcarrier is considered to be "in the grasp" of a defender and his forward progress has been clearly halted; when a forward pass hits the ground or lands out-of-bounds without being caught; when a ball is dropped (or "fumbled") and lands out-of-bounds or in the arms of a downed player; or when a kicked ball leaves the field of play. However, the game clock continues to tick during most of these scenarios.
Play Calling (or Deciding What to Do): Between plays you'll usually notice each team gathered in a tight circle on its designated side of the field, where players will go over strategies and formations for the next play. These tight circles are known as huddles, and I've always thought they were one of the cutest aspects of the game. You don't usually hear words like cute tossed around regarding football players, but what can I say? Ever since I was a kid, I thought it was adorable the way these brutes lined up and circled their wagons to discuss their next moves. (In some college programs, players even hold hands in their huddles -- I mean, how cute is that?!)
The offensive team will form its huddle about 10 yards from the ball, where for about 10 to 15 seconds the quarterback will bark out a play and offer general words of encouragement (or constructive criticism that may or may not include a few carefully chosen expletives) to his teammates. Frequently, the play will be decided by a coach on the sidelines or up in a box who will send in his call through a series of hand signals, a set of hollered codes -- or, these days, via radio transmission from a headset to an earpiece strategically placed inside the quarterback's helmet.
At the same time, the defensive team will form its own huddle, just beyond the ball on its side of the field -- as close as possible to the goings-on of their opponents. (Ah, the better to possibly hear what's going on in the enemy camp.) Here, too, a defensive captain will communicate to his teammates how the coaches want them to approach the next play.
In some situations, most notably toward the end of each game when time is running out and trailing teams move into "hurry-up" mode, teams might deploy a no-huddle offense. This means that plays are called at the line of scrimmage when players are already in formation, instead of in the huddle. This also means, logically, that defensive teams must go without a huddle as well, because the quarterback can start the play whenever he wants.
Actually, let me amend the "whenever he wants" part of this last statement. The quarterback can't actually start the play until his offensive unit has lined up in formation and until the defensive unit has retreated to its side of the ball. Plus, there's the 40-second play clock within which the offense must start each play to avoid being penalized for a "delay of game."
In any case, play resumes after the ball is spotted (placed down) by the linesman and the two teams break from their huddles and line up in formation on either side of the ball, which becomes known as "the line of scrimmage."
Typically, one of the offensive linemen (the center) will line up in a crouch over the ball and snap it between his legs to the quarterback on an agreed-upon signal. The quarterback can then hand it off or make a forward or lateral pass to an eligible back or receiver, or run it himself. A forward pass may only be made from a position behind the line of scrimmage, and only during a play run from scrimmage. (That is, a forward pass cannot be attempted on a kickoff or punt return, or following an interception or fumble recovery.) A lateral pass (a backwards throw from the QB to a running back or wide receiver) may be made at any time, from anywhere on the field. Similarly, a handoff may be made at any time, from anywhere on the field, provided that the initial ballcarrier hands the ball off to a receiving ballcarrier from a position away from the downfield side of his body. During a play run from scrimmage, the initial ballcarrier can hand the ball off from either side of his body, provided he has not yet crossed the line of scrimmage.