Wiki has a good overview.
http://en.wikipedia.org/wiki/Australian_rules_footballBasically, two teams, 4 posts at each end.
Each team tries to kick the ball (which is oval shaped) through the posts at the opposite end to which they're defending. Kick it through the middle 'zone', you get a goal, worth 6 points. Kick it through the two smaller 'zones' to each side of the larger one, you get a point, worth (obviously) 1 point. Team with the most points at the end of the 4 quarters wins.
That's the aim. (With much simplification)
There are plenty of intricacies, as there is with any sport.