Beers fall into two broad categories: Those that are produced by top-fermenting yeasts (ales) and those that are made with bottom-fermenting yeasts (lagers). There are hybrids, but that's another discussion.
Ales came first, when brewers weren't exactly sure what role yeast played. Because ales were unstable, brewing ceased in warm weather and brewers would store reserves in as cool or cold an environment as they could find. Brewers storing their beer in very cold Alpine caves found that their beer was more stable because the yeast had sunk to the bottom.
We won't go into the evolution of this yeast, but this storage (lagerung in German) naturally selected bottom-fermenting yeasts. Operating at colder temperatures these yeasts worked slower, producing beer more attenuated, cleaner, rounder and less fruity than ales. Fermentation took one to three months.
Ales include everything with ale in the name (pale ale, amber ale, etc.), porters, stouts, Belgian specialty beers, wheat beers and many German specialty beers. They generally have a more robust taste, are more complex and are best consumed cool (50F or a bit warmer) rather than cold.
Lagers include pilseners, bocks and dopplebocks, Maerzens/Oktoberfests, Dortmunders and a few other styles found mostly in Germany. They are best consumed at a cooler temperature than lagers, although anything served at less than 38F will lose most of its flavor.
American variations on the pilsener style dominate the U.S. beer landscape, but unlike 20 years ago there are plenty of other choices today. Writing in the first edition of his "The Pocket Guide to Beer" in 1982 (the current edition is No. 8),
Michael Jackson described ales and lagers as the "red wines" and "white wines" of the beer world. He concluded: "The popularity of the original pilsener was well deserved, but its renown is ill served by the many brewers in different parts of the world who have used indifferent imitations to try to create a single international beer style at the expense of more characterful regional specialties. It is as though the whole world were to drink Rhine wines and forget about the very existence of Burgundy or Bordeaux. The 'whites' of the beer world are more stable and consistent, but the top-fermenting yeasts endow the 'reds' with great personality."
