Alpha acids, pleasant bitterness you want in your beer, are in inactive form in hops. They need to be isomerized to taste the way it should. This takes time and temperature, around an hour of boil to convert all of it.
Aromatic components of hops needs only to be washed out. But they degenerate and evaporate with boil, so the shorter you keep them hot, the more you will get. Ultimate is dry hopping, with no time hot at all.
Some components that brings taste needs to be boiled out of hops. This takes from 10 to 30 minutes - after that time, they begin to degenerate and evaporate, too.
That's why there are three traditional times to add hops:
- 60' - bitterness, but almost no taste or aroma
- 30' to 10' - taste, low to moderate contribution to bitterness and aroma
- 10' to 0' - aroma, low contribution to taste, almost no bitterness