It is a game for two players who control rival teams; they are limited to 11 players on the pitch though a squad can have up to 16. The basic objective is to score touchdowns by getting the ball into the opponent's end zone.
This is achieved, primarily, by bashing the opposition out of the way, and preferably injuring as many of them as possible. In a league format, your players gain experience points and level up for injuring the opposition (also for scoring, and completing passes).
The rules are quite complicated as to how this is actually achieved - there's a variety of skills and lots of dice involved. The full rules are various places on the internet (legally so) - [here] for example.
Basically, though, players have four stats: Movement is how far they can move Strength is how well they block Agility is how well they handle the ball and dodge Armour is how well they take a punch.
Whenever you want to do anything other than move you roll a d6. Stats and skills make your chance of succeeding at the d6 roll better; either lowering the target number or giving you rerolls. You can also buy extra team rerolls. Sometimes players get knocked down by other players. Then you roll 2d6 and compare to armour. If the roll is higher, you roll more dice to see how badly they got squished.
All this dice rolling made the game an excellent candidate for computerisation, and various attempts have been made. The most recent and now actually fairly successful since most of the bugs seem to have been splatted now the second expansion's been released, is [this]. It's fun, but the AI is terrible.