The grain is usually barley, but can be other grains. Hops is a vine. The flowers are cone shaped and green. The flowers are the part that is used in beer making. They are what gives the beer its aroma and flavor.
Yeast is the enzyme that breaks down the sugars into alcohol and carbon dioxide.
Today you can buy these ingredients in a kit at a homebrew supply store. In these kits the grain has already been malted and flavored with hops. This has been condensed into heavy syrup. The beer making kit is good for making your first few batches of homebrew. Brewers yeast can be purchased at the homebrew shop.