All beer can be classified as either a lager or an ale. The differences begin during the brewing process. Whether the beer is an ale or lager is defined by the type of yeast used in the brew and the temperature at which fermentation takes place. Ales are brewed with top-fermenting yeast which allows for rapid fermentation at warmer temperatures; Lagers are brewed with bottom-fermenting yeast which ferments more slowly and at colder temperatures