Ingredients

Beer is made from four ingredients: barley, hops, water, and yeast

Barley:
A grain that is used to give beer flavor and color.  The barley can be kilned, roasted, or toasted to varying degrees.  Some beers use just one type of barley, while others incorporate several in order to give the beer a more complex flavor.  Some types of barley create caramel flavors, some create biscuit or bready flavors, and some create chocolate or coffee flavors.

Hops:
Beers need to be balanced.  The sugar that barley imparts to beer is very sweet, to counteract that, hops are used to add bitterness.  If hops weren't used, the final product would be very sweet, perhaps undrinkable.  Hops grow on vines, and when they bloom, the result is a flower, called a hop, which resembles a pine cone but it green and leafy.  There are many different types of hops.  Depending on how they are used in the brewing process, they can add bitterness or even flavor and aroma to the beer.

Water:
Most homebrewers don't worry too much about water, as long as it's clean and pure.  But, most big breweries are very careful with their water.  Water contains a lot of minerals.  If the profile of the water is too high in one area, sometimes additives will be added.  Additives could include salts or different types of buffers.  Some beers produce better results with a particular water profile, so some breweries use different types of water for different types of beer.

Yeast:
When it comes to beer, or even wine for that matter, yeast really is the main component.  And just like bread, you can't make beer or wine without yeast.  The yeast will consume the sugar in the liquid, and as a result, will produce alcohol.  It doesn't matter how careful the brewer was with choosing ingredients or using good, clean equipment, because the yeast will ultimately finish the job.  And for the most part, brewers have no control over this step.  They just sit back and let Mother Nature do her job.