Fundamentally beer is the product of the alcoholic fermentation by yeast of extracts of malted barley. Whilst malt and yeast contribute substantially to the character of beers, the quality of beer is at least as much a function of the water and, especially, of the hops used in its production.
Barleystarch supplies most of the sugars from which the alcohol is derived in the majority of the world's beers. This starch is enclosed in the cell wall and proteins within the barley, and these wrappings are stripped away in the malting process (essentially a limited germination of the barley grains), leaving the starch preserved.