This questions is actually a bit tougher to answer than what you might think. Traditionally beer was made out of countless things. Some might find that to be a surprise as most people declare that “traditional” beer can only be made with grain or malt, hops, water, and yeast. This belief really only dates back to the 1490s and officially to 1516 when Bavaria’s reigning Duke Wilhelm IV declared that the Reinheitsgebot take place over all of Bavaria. Reinheitsgebot literally means “purity law” and it was the first ever food safety law. This law actually helped Germany and the connecting areas of Bavaria become renowned for their superior quality in beer.
Beer dates back to as early as 6,000 BC and the first real proof we have dates back to 3,500 BC in Egypt. Dogfish Head currently makes a beer called Midas Touch, which I will let their website explain,
This recipe is the actual oldest-known fermented beverage in the world! It is an ancient Turkish recipe using the original ingredients from the 2700 year old drinking vessels discovered in the tomb of King Midas. Somewhere between wine & mead; this smooth, sweet, yet dry ale will please the Chardonnay of beer drinker alike.
The beer is made with honey and saffron among other things. Essentially beer really only needs a few things to work. You have to have some type of sugar, which is usually derived from malt a.k.a barely grain. Water is necessary to get the whole ball rolling becasue the process of making beer involves soaking the grain in hot water (140-160 degrees Fahrenheit) for a set amount of time. While the grain is soaking in the hot water enzymes in the grain are activating and turning the starches stored in the grain into sugar. On a side note beer is about 90% water on average. Sugar is essential becasue the next ingredient needs it to live.
The sugar is eaten by yeasts that have been specially cultured for the beer making process. The yeast eat the sugar and turn it into three things; carbon dioxide, heat, and alcohol. If you wanted to, you could really stop here as you technically have beer at this point. The problem is that the yeast take the time to turn the sugar into alcohol and other microorganisms also like beer. Once the yeast have enough time to turn sugar to alcohol, those microorganisms cannot survive in an environment with alcohol. To give the yeast enough time to do thins, hops has been the ingredient of choice to help preserve the beer. It also helps it have a longer shelf live and helps balance the taste of the malt in the beer. The malt is sweet and the hops are bitter, together you have something that tastes wonderful.
So the German’s Reinheitsgebot does have all of its bases covered for making beer. However there are a lot of other things that can go into a beer to add to it. Think of the Reinheitsgebot in terms of pancakes. It allows you to put pancake mix, water, milk, and syrup into your pancakes. Sure you can have wonderful tasting pancakes with those ingredients but what happens if you want some whipped cream or chocolate or strawberries. I think you get where I am going here.
In the United States the Reinheitsgebot has really never been something that has been followed. During the Colonial times spruce branches were a common ingredient in beer. The were mostly used as a substitute for hops but added a distinct flavor of their own. Today breweries are producing beers with all kinds of extra ingredients in them. Fruit beers, spiced beer, etc are all mass produced and provide something new and different. Our friend from the pancake ingredient list, syrup, is also used in many different beers. So the question of what is beer made of doesn’t really have a true answer. I suppose that all/most beers have the following things in common; malt, hops, yeast, and water. After that it is really anyone’s guess.