Contrary to what David Spade says in Tommy Boy, hops are not malted. Hops are used as a bittering agent to balance against the malty sweetness of beer, and also contribute aroma and flavor.
Without getting overly technical, the resins found in hops have two types of acids: alpha and beta. Alpha acids isomerize during the boil, and contribute bitterness to the beer, measured in International Bittering Units (IBUs). The longer you boil the hops, the more IBUs you get. Beta acids, however, do not isomerize, and primarily contribute aroma and some flavor to the beer. Hops are rated with regard to their alpha-acid percentage, indicating its potential for providing bitterness. Higher AA% = more potential for bitterness.
There are almost as many different types of hops as there are types of malts. Hallertauer Mittelfruh, for example — frequently touted by celebrity fanboy, Jim Koch, as the signature hop in Sam Adams — is often used for adding aroma to European lagers and pilseners. Meanwhile, Cascade hops have become virtually synonymous with American Pale Ales. You can easily identify Cascade by its fragrant citrusy grapefruit aroma. Just pour yourself a Sierra Nevada Pale Ale. Trust me, you’ll smell it.
For a great summary of different types of bittering and aroma hops, check out Norm Pyle’s hop FAQ.