Hops: The "spice" in beer. They are cultivated flowers (Humulus lupulus), used to preserve and flavor beer. The bitterness of the hop is used to balance the sweetness of the malt, and the essential oils add a taste and aroma which cannot be achieved by using any other plant. The hop plant is a perennial twining vine, which will grow in almost any climate given enough water and sunlight. The flowers are usually dried before use. Hops, just as grapes used in wine, are varietal. Certain varieties are selected for their bitterness, while others are chosen for their distinctive aromas.
Yeast: The single cell organism that metabolizes malt sugars into alcohol and carbon dioxide. There are different species used to produce different styles of beers. The most basic distinction in yeasts is their various migration tendencies within the barrel during fermentation, e.g. those that rise to the top of the barrel during fermentation create ales while those that stay at the bottom create lagers. The species of yeast chosen by a brewer is also fundamental in determining a beer's flavor and aroma.
Water: Contrary to what one might think after throwing a couple back, beer consists mostly of water (90%). Consequently, the quality of a brewer's water source is very important. Hard water often contains minerals that act as catalysts in fermentation, resulting in distinctive complexities, while soft water's lack of minerals promotes roundness and smoothness.
Malt: It is grain (usually barley or wheat) that has been soaked in water, germinated, and then dried in a kiln. During germination, enzymes emerge that transform the seeds into sugars, which will eventually be fermented into beer by yeast. The variety of grain, the amount of time which it is given to germinate, and the temperature at which it is dried all determine the taste and body of a beer.