AAU stands for Alpha Acids Units. All hops have a range in which they produce a certain amount of bitterness. Each year that hops are grown based on rain values or a dry season. That number can change.  Low is 2-4 percent; medium is 5-7 percent; high is 8-12 percent and with all the new breeds it gets higher, check out Polaris

Used to add bitterness, flavor, and aroma to the beer. In general, hops added at the beginning of the boil are for bitterness and hops added around the 30 min mark are for flavor and the last 5 minutes or so are for aroma. The thing to think about is the longer the hop boils the more bitterness is extracted.

A soft resin in hop cones. When boiled, alpha acids are converted to iso-alpha-acids, which account for 60 percent of a beer's bitterness.