Basically, the earlier in the boil the hops go in, the more bitterness is extracted, and the more flavour (from the oils) is boiled off.
So for a normal pale ale, it's good to have a mix of bittering, late and aroma hops to give a good balance between the different aspects.
Even with the aroma hops (added very late in the boil, or even at "flameout" whilst it cools), some of the aroma will come out of the brew with the CO2 that gets released during fermentation, hence why there is mileage in dry-hopping after the primary fermentation is over or adding a hop tea directly before bottling/kegging.