" A fine beer may be judged with only one sip, but it's better to be thoroughly sure. - Czech Proverb
Beer is the world's oldest and most widely consumed alcoholic beverage and the third most popular drink overall after water and tea. It is produced by the brewing and fermentation of starches, mainly derived from cereal grains such as malted barley, although wheat, corn, and rice are also widely used.
Most beer is flavored with hops, which add bitterness and act as a natural preservative, though other flavorings such as herbs or fruit may occasionally be included. Alcoholic beverages distilled after fermentation or fermented from non-starch sources such as grape juice (wine) or honey (mead) are not classified as beer.
The most common method of categorizing beer is by the behavior of the yeast used in the fermentation process. In this method of categorizing, beers using a fast-acting yeast which leaves behind residual sugars are termed "ales", while beers using a slower-acting yeast, fermented at lower temperatures, which removes most of the sugars, leaving a clean, dry beer, are termed "lagers". Differences between some ales and lagers can be difficult to categorize.
Here are the general characteristics of the two main types, which are further subdivided into several categories:
Ales generally use top fermenting yeast. This means that the yeast floats on the surface for the first few days and then settles on the bottom. Ales ferment at warmer temperatures (65-75°F) over a relatively short period of time. Ales have a wide variety of colors, aromas, tastes and strengths. Porters, stouts and Weizens are subcategories of Ale.
Lagers use bottom fermenting yeast, which does not float to the surface before settling. Lagers ferment at cooler temperatures (46-55°F) and are stored over several weeks. The cooler temperature and aging impart smoother, more subtle flavors and aromas than most ales. Your common American Pilsners, such as Miller, Coors, Rolling Rock, etc., are a sub-category of Lager beer.
There is a third type of beer far less common than Ales or Lagers, called Lambic. True Lambic is only brewed in the Payottenland region of Belgium. In Ales and Lagers the yeast is specially cultivated for fermentation. Lambic is exposed to open air instead, and fermented by naturally-occurring wild yeast.