** This question was asked by an individual looking to brew beer at home. Even though it is geared towards brewing, it is still a good description of the main ingredients of beer.
In its most simple form, beer is made of four basic ingredients: Water, yeast, Malt and hops.
As is the case with the human body, the most abundant ingredient in beer is water. Over 95% of your brew is made up of water. With such a large amount of your final product coming from a single ingredient, you can imagine how important it is to use good water.