Lagers are brewed with "bottom-fermenting" yeasts at much lower temperatures (2 to 10 °C) over long periods of time (months). This process is called "lagering". Lagers include bocks, doppelbocks, Munich- and Vienna-style, M?rzen/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.

Source : FAQ of alt.drinks.beer