Hops are used both for adding bitterness to beer, and for adding distinctive aroma. Brewers frequently use one variety for aroma and another for bittering.
A few common varieties of hops
Cascade -- a US variety, extremely popular. Has a citrusy aroma. Found in Sierra Nevada Pale Ale.
Fuggles -- UK variety, gentle, grassy aroma.
Tettnanger German, spicy. Used in Sam Adams Octoberfest.
Saaz -- Characteristic Czech hop. Used in Pilsener Urquell.
Hallertauer -- German, mild, herbal. Found in Sam Adams Lager.