This article is from the rec.food.drink.beer FAQ, by John Lock with numerous contributions by others.
Lagers are brewed with "bottom-fermenting" yeasts at much colder
temperatures, 35-50F (2-10C) over long periods of time (months). This
is called "lagering". Lagers include bocks, doppelbocks, Munich- and
Vienna-style, Maerzen/Oktoberfest, and the famous pilsners. Pilsner
beer originated in the town of Pilsen, now in the Czech Republic and
was the first non-cloudy beer. Most popular beers produced by the
large North American breweries were originally of the pilsner style.
These have diverged a great deal from the original style and succeed
now by the force of the mass-marketing prowess of the brewers rather
than any remarkable qualities of the beers themselves.
 
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