well, it's insanely complicated. the story is funny though. football, american football and rugby were all the same sport up until the mid 19th century, when they started differentiation from each other. when you look at old american football film (i mean black and white from the early part of 10th century), it's MUCH more like rugby. then it slowly changes, bit by bit, into the game it is today.
a typical game might have 6 scores in total, and action moves in one-direction at a time, though it can switch on a dime. the basics are simple, move the ball down the field and score. you have 4 chances to go 10 yards, and if you don't, you lose the ball. every 10 yards you get a fresh set of 4 chances (or downs). the defense tries to stop you, or get the ball away from you, so their team can take over. positions are highly specialized in a way they are not in football. the game is super violent, but not in a bloody, ears-come-off way like rugby, in a more car collision, broken bone sort of way.