Beer has been a part of world culture since the literal beginning of civilization, as in Mesopotamia, where they grew wheat for bread...and beer. In addition to wheat, barley, corn, rice, millet and sorghum are all used to make beer, and together with differences in the brewing process, create the plethora of beers available around the world today.

Basically, beer is created from the fermentation of the cereal used with water, barley, yeast and hops. The most important factor in brewing beer is the purity of the water—rigid purification standards amongst breweries ensure that the highest quality is used, and ensuring a quality product. Barley brings the fermentable sugar to the beer. Turned into malt, the kernels are crushed at the brewery then added to the heated water, where they begin to break down the starch of the cereal into sugar. This step is also where additional products are added, such as those to make oatmeal beer.

The mixture is then strained, with the liquid (or wort) running off into a brew kettle. Boiling in the kettle sterilizes the wort and extracts flavor, aroma and bitterness from the hops. After a period the hops are removed and the wort moves into a cooling tank, then to a fermenting holding place and yeast is added. The type of yeast varies according to the desired beer. Fermentation can last up to ten days. At the end of the fermentation process, with the yeast removed, beer emerges.

Information gathered from:
http://www.tourismvictoria.com/Content/348.asp
http://encarta.msn.com/encylopedia_761572428?Beer.html