Bottom fermentation or bottom fermenting is a term that describes the manner in which lager yeast tends to collect on the bottom of the fermenter and conducts its fermentation, as opposed to top fermenting ale yeast, which conducts most of its fermentation on the top of the beer.  Bottom fermenting lager yeast strains prefer a low fermentation temperature range that is typically between 40° F and 55° F, but varies between strains.