Hops are an essential element of all beer and have two basic purposes:
Hops added early during the boiling of the wort will bitter the beer and hop added at the end of boil will provide aromatic properties.
When using hops to bitter beer, brewers want to know how much hops they need to achieve a particular IBU. This is even more complicated given the vast variety of hops available. Therefore a measuring unit was developed to determine the amount of Alpha Acid in hops. The higher the AAU, the more bittering properties the hop has.