@VijayKapoor,
Oh, boy.
American football has some fairly complicated rules but the main idea is to get the ball from the end zone (your team's side) to the opposite end (the other team's side). Scoring is either through a touchdown (6 points), an extra point field goal, which is a kick through the goal posts and can only come after a touchdown (1 point), an independently kicked field goal (3 points), and a conversion, where the offense may attempt to re-advance the ball into the opponent's end zone (2 points).
The guys, I am sure, will chime in with more information.