Malt is the source of sugar and this sugar is converted by the yeast into two by-products: carbon dioxide and alcohol. Malt is grain that has been prepared (i.e. malted) for use in brewing beer and the grain most commonly used is barley. Other grains are used for different styles of beer (Weizenbier is made from wheat malt). The amount of alcohol produced in the brewing process is directly related to the amount of malt added to the wort (what beer is called before it is beer). Home brewing enthusiasts can add malt (sugar) to their wort using several types of malt including malted grains, malt extract syrups and dried malt extract (or DME).
Hops are added to the beer wort in relatively small quantities. Hops are available in several forms including plugs, pellets, leaf and fresh whole hops. Most homebrewers use pellet hops because they are the easiest to use and store and are the most widely available. Depending on when hops are added the the wort they provide bitterness, flavor or aroma.
Yeast is available both dry and as a liquid. As stated before, yeast converts the sugars present into alcohol and carbon dioxide. The carbon dioxide escapes during the fermenting process. When the fermenting process is complete the sugars are mostly gone and you have beer that is flat. By adding corn sugar to your beer as you bottle it you provide just enough sugar for the yeast to again ferment (in the bottle). This produces enough carbon dioxide to carbonate your beer.
Water makes up the bulk of your beer ingredients! The most important thing about water is that you use good water. You can use tap water if it tastes good. If your water does not taste good then purchase some bottled spring water and use that.