Having already written on the other three aspects of beer (water, grains, and yeast), I can now address the last: hops. Among beer ingredients, hops are really the most expendable. No matter how strange this sounds, the other three ingredients have always been necessary for making beer. The hop is more recent phenomenon. Although use of hops in beer can be found sporadically in the history of beer, it did not gain wide currency in beer until the 15th or 16th century A.D. Hops add bitterness and flavor to a beer, but how were beers bittered/flavored before their popularity?
Gruit, anybody? Juniper, anyone? Various mixes of herbs and spices were used in beer to add bitterness or flavors. One ingredient that was used to bitter was Irish moss (guess where this was used). Even though this ingredient is still used, albeit for clarifying a beer, it is not used in the same bittering capacity as former times. So since the hop was king, what else did it do?
Like grains, water, and yeast, hops can add various flavors to a beer. One primary use of hops for beer is to bitter. These are often added early in the brewing of a beer and the flavors recede to the background. In these cases, hops are for bittering purposes. However, a second (or numerous) addition of hops near the end of the boil adds hop aroma and flavors. Some brewers even add hops during fermentation; this is called dry hopping.
Another thing that hops do is to add longer life to beer. The acids and oils in hops act as a preservative (and even an antibacterial). Some styles even came about because the addition of hops were necessary to preserve the beer longer. IPA is a perfect example. I intend to talk about that elsewhere. Suffice it to say that hops are the latecomer in beer history, but they came with a vengeance. Interestingly, some brewers are beginning to revisit Juniper and other sources to bitter beers. I don’t know if history will be cyclical in this case (I suspect it won’t), but it will be interesting to see what these beers will be like.