Water comprises more than 90% of beer. Today, almost any water can be chemically adjusted to create the exact style of beer desired.
Barley is a cereal grain of three major forms – two-row, four-row and six-row. Two-row and six-row are used for brewing beers. Barley must go through malting (bringing grain to sprout roots) to brew beer. Some are then roasted to different degrees to bring out various colors and flavors that are used to create different colors and flavors in the beer.
Hops are cone-like flowers. They are used as a preservative and for their essential oils that add flavor and aroma to balance the sweetness of the malt.
Yeast is the real hero that converts the malt/water/hops mixture into beer. It is a fungus that converts sugars into alcohol and carbon dioxide found in beer, through a process called fermentation. Some yeast ferment at the "top" of the fermentation vessel, while some ferment at the "bottom".