Welcome to Homebrew Extreme - Your beer brewing home! Want to brew some beer? We'll show you how. Everything you need to know to brew great beer, at home.

Beer, as we know it today (actually since around the 14th century) has been made primarily with 4 ingredients, Water, Malted Barley, Water, and Hops. Hops offset the sweetness from the malted barley by contributing significant bitterness to beer, as well as a pleasent aroma.
Hops, a very prolific vine, botanically related to the hemp or marijuana plant, grows very rapidly over the summer months. Because of it's heartiness, it can be grown most anywhere, however, the US and Europe are it's most common origins.
One use of hops in beer is Aroma and Flavor. These hops are called finishing hops. Finishing hops generally placed in the brew pot for the last 1 to 20 minutes of boiling depending on the desired effect of the hops. When selecting finishing hops for a particular beer, or beer style, many things are taken into consideration. Geographical origin is important. For example, you wouldn't want to brew your fine English Brown Ale with Noble German Hallertauer hops. Well, you certainly could, and it could make a fine beer, but it would not meet the conditions to be called an English Brown. So, when selecting hops, always consider the style and origin. The second consideration is flavor. Various hops impart various flavors, so selecting the appropriate hops for you brew is critical. An example here is that of Cascade hops. Cascade is from the Cascade mountain region of the U.S. and is known to impart a pleasant floral, citrus flavor to beer.
The most important purpose of hops in the brewing process is bittering. During the malting process, the starches in Barley are converted to Sugar which makes it extremely sweet. Bittering hops offsets this extreme sweetness by infusing bitterness.
The amount of bitterness in hops is determined by the amount of Alpha Acids in contains. In the brewing world, this is measured in IBU's, International Bitterness Units. A given style of beer might call for a specific level of bitterness. For example, a Belgian Style Ale calls for 15-25 IBU's of bitterness.
Common Hops Chart:
(Name, description, origin, usage)
Amarillo Recently developed Aroma hop US 8-11%
Cascade Aroma, fragrant citrusy hop US 4.5-6%
Centennial Aroma/Bittering, similar to Centennial, not as citrusy US 9.5-11.5%
Chinook Bittering, high alpha acid, smoky character US 12-14%
Columbus Bittering Pungent common in high hop varieties US 12%
Fuggle Aroma, common in English ales, unique character UK/US 1.5-2%
Kent Golding Aroma, Very common english aroma hop spicy UK 4-6%
Hallertau Aroma, Mildy spicy, common in lagers Ger. 3.5-5.5%
Northern Brewer Bittering, common high alpha, strong fragrant US/UK/Ger 8-10%
Saaz Aroma, primaily used in Pilsners, very clean Czech 3%
Tettnang Aroma, fine, pure, mellow aroma, lagers and wheat beers Ger. 4-5%
Willamette Aroma, fragrant, spicy wood aroma, American beers US
There are literally dozens of other, less common hops. When available, homebrewers should experiment with as many varieties as possible. In this brewers humble opinion, nothing impacts the quality and taste of beer quite like the hops.