The use of hops is one of, if not the most, important differences in the production of beer from the production of other alcoholic drinks. Without the bittering effect of hops, beer would be nothing more than a low alcohol wine made from barley (in fact, with a more alcohol-resistant strain of yeast and a slightly higher amount of fermentables, it would be a wine). Hops are used for three basic reasons: bittering, flavouring, and aroma. They also have somewhat of a preservative effect, although the extent of this effect is debated. In the days before refrigeration, shipments of beer on long voyages would often sour or spoil, due to bacterial or fungal infection or simply due to aging. Breweries often would steep herbs and plants in the unfermented wort to inhibit this spoilage and stabilize the flavour of the beer. Hops simply won out due to the popularity of the flavor, and the benefits it provided, such as improved head retention, clarification, and the counteraction to the sweet, sticky taste beer had otherwise.
Some examples of hop varieties are Hallertau Mittelfrueh, Tettnang, Spalt, Saaz, Mt. Hood, Perle, and Cascade, and they are available in whole form and pellets.