So what are the ingredients in beer? Beer is generally understood to be beverage comprised of Barley, hops, water and yeast. However, there are many things can be substituted in the cereal side of things. So, along with your Barley you might have some wheat or you might have rye or you might substitute it entirely for something like a gluten-free beer.
There are have been throughout history laws that have actually been made to say to based upon the purity of bee that limit the brewer to those specific ingredients, water, barley, hops an yeast.
However, also throughout time man fermented beverages have been made wit almost any ingredient that you can think of, and that continuous to this day with the innovation in craft brewers. So, for instance there is a brewer, Stillwater who just made a Saison that included the ingredients of whole juniper bushes. Really you can get to anything in terms of what the ingredients are in beer, but it's basically, barley, hops, water and yeast.