11 men on the field at a time (on offense, you need at least 7 people on the line of scrimmage, and a Quarterback to receive the snap to start every play).
Players on offense are broken down into many positions, the main ones are Quarterback, Running back, Full back, Wide Receiver, Tight End, Tackle, Guard, Center.
Players on defense are broken down into 3 levels, Linemen, Linebackers, Defensive Backs. Lineman normally consist of Ends and Tackles. Linebackers usually consist of Inside and Outside Linebackers. Defensive backs usually consist of Cornerbacks and Safeties.
Teams also use kickers and punters on special teams.
The object of football is to advance the ball downfield by passing and running.
There are multiple ways to score, touchdowns (6 points), Point After Conversions (1 point), 2-point conversions (2 points), Field Goals (3 points), and Safeties (2 points).
A touchdown occurs when the ball is advanced to the endzone. After this, a team can elect to kick a point after conversion, or run a play from 2 yards out to try to pick up 2 points.
A Field Goal occurs when a team kicks the ball through the uprights.
A Safety occurs when the defensive team tackles the ball carrier inside their own endzone.
The field is 100 yards long.
The current World Champs are the New England Patriots, though...