Speaking from Australia, what I understand is that the offence are trying to carry the ball a certain distance.. uh like first down and . Similar to League, the offence needs to get the ball that distance in five or six(?) possessions. Of course ultimately the goal is to score a touchdown (try) but downs are the 'stepping stones'.

Also you'll find that most of the grabbing and what not occurs on the Line of Scrimmage, the tackles that are important in Union and League don't happen as much in American Football. There are tackles applied, don't get me wrong, but particularly on the wings and in broken play knocking WRs and so on out of bounds can and does suffice.

Personally I liken American Football to the superior Union code of Rugby, the game is very stop and start with a lot of tactical jousting between the offense and the defense. It's a toos up over which code is better to watch, it's far easier for me to tell what is going on in American Football, but I find Rugby Union games enthralling to watch.

Or just have a look at Lia's link and not read my textual spaghetti.