OK, all commercially grown hops are analyzed for their alpha acid level. This rates, for all intents and purposes, the "bittering power" of the hop. It is easily looked at as a straight linear scale. A hop with an alpha of "8" is twice as bitter as a hop with an alpha of "4", but only half as bitter as a hop with an alpha of "16." This means that one ounce of an "8" hop, or 2 ounces of a "4" hop, or one-half ounce of a "16" hop would all impart the same bitterness if they were all boiled for the same length of time.
Hop (to put it simply) impart their maximum bitterness after being boiled for 45 to 60 minutes - after that time, you cannot "suck out" any more bitterness from those hops; partial bitterness is extracted after a hop is boiled for 15 to 30 minutes, and little bitterness if they are boiled for less than 10 minutes. Now this is straight "lingering" bitterness. That means that you can have the most powerful hop in the world and if you throw it in for the last 5 minutes of your boil, YOU WILL BARELY TASTE THE BITTERNESS OFF THAT HOP! While if you were to boil it for sixty minutes, that would be a very bitter beer. Think of it as the bitterness being slowly sucked out of the hop as the boil works on it.
OK, so why add hops for the last few minutes? Or for the last twenty minutes? Or anywhere in-between? Well, hop additions for that last 25 to 15 minutes of the boil add deep "back" flavors into the background of the beer - flavors (not bitterness!) of the actual herbal/flowery/spicy FLAVORS that the hop originallynhad - this is different than bitterness. Hops added for the last ten to fifteen minutes of the boil add fresh "zestier" flavors, more "up-front" and livelier - and hops added for under five minutes - that's the last five minutes of the boil OR LESS - add that flowery/citrus/spicy SMELL into the beer's profile. Those hops add aroma.
Most of the recipes I give out through my store have three or so additions of hops - (some have six or seven) to take advantage of this great layering effect. So, be aware that nothing is limited in scope. Hops added at 25 or 20 minutes remaining in the boil add that deep "back flavor" but also add to the overall bitterness somewhat. Not as much as if they were boiled for 60 minutes, but the boil has definitely extracted out some of their bitter qualities along with their flavor.
And what about hops added for the last 7 or 9 minutes of the boil. These will add some aroma, some flavor, but not very much bitterness - additions such as these make beers very complex and full-flavored, as many beers should be!