Before we start... if you are sitting comfortably... lets have a quick lesson on the basics of beer ingredients.
Hops: Hops are used in the brewing process for their bittering flavours and aroma. For example, a traditional English brew will use a third more hops than beers of the lager variety. More than one type of hop can be added to give a specific characteristic, and sometimes even whole hops are added to the beer at the brewery (a process called dry hopping) to give extra aroma to the beer.
Barley: Barley is used to give a beer its body and strength. Malting quality barley, which has a lower nitrogen content, should be used. Too high a nitrogen level can cause cloudy beer. Barley is converted into malt by germinating the barley in water, a process that breaks down the complex carbohydrates in the grains. The germination is stopped by drying, and then crushing the grains to give malt.
Water: Water is vital to the whole process. Regional water varieties, combined with artificial treatments, give a beer its distinctive regional tastes.
Yeast: There are many varieties of brewing yeast, and many breweries are very secretive about the specific cultures that give their brews their distinctive tastes.