Yep Greg is on the money. Hops. It comes in different varieties which give different flavours. They actually make the beer bitter as it would be very sweet from the sugars present in the barley (and if home brew the fermentable sugars added) that the yeast consume to make the alcohol. Actually Bourbon and beer are very similar for a while,minus the hops. They are both brewed to form a wort using yeast to ferment and make the alcohol. the bourbon is brewed from corn instead of barley(some crap mass produced beer also uses corn as it is cheaper). The Bourbon is then passed through a still. The beer is pretty much made at this point.
Different beers are made from heating the barley for different lengths of time. Stout uses barley roasted till it darkens. A larger is roasted for shorter. Good beer uses no preservatives, is NOT filtered and is found usually on tap at my fridge door. And here is the good part......... its about 50 cents a Schooner.
Sorry I am a home brew addict.