When a beer is fermented at colder temperatures (41°F–50°F, 5°C–10°C) the yeast work less vigorously and produce a lighter and cleaner product. This cooler fermentation is known as lagering and is used for lagers.
This cooler ferment does not create the traditional "crust" on top of the wort which a vigorous top fermentation produces. Instead, the yeast clump together lower down in the fermenting beer.
Bottom fermentations can last 10-14 days, unlike a top fermentation which generally finishes in 3-4 days.