The branch of the beer family tree — ale or lager — corresponds to the type of yeast used to ferment the beer. You have ale yeast and lager yeast, and these types of yeast, in turn, typically dictate the temperature at which the beer is fermented. Ales are traditionally fermented at warmer temperatures (55 to 70 degrees Fahrenheit, 12 to 21 degrees Celsius), while lagers are typically fermented at cooler temperatures (38 to 50 degrees Fahrenheit, 3 to 10 degrees Celsius).
The cooler fermentation and aging temperatures used with lager yeast slow down the yeast activity and require a longer maturation time. The cold environment inhibits the production of fruity aromas (called esters) and other fermentation byproducts common in ales. This process creates the lager’s cleaner taste. Long aging (or lagering) also acts to mellow the beer.