Rules of Football
There are two purposes in a game of American football: one is to gain field so your team can score points with the football, and the other is to defend your team's territory. With that in mind, each of the two teams plays strategic offense to move the ball and score, while working hard on defense to stop their opponents from doing the same.
Teams play four 15-minute quarters, at the end of which the team with the most points wins. The clock stops for various plays, and there's a halftime break between the second and third quarters, so it usually takes between three and four hours for a game.