From Wikipedia, the free encyclopedia. Lager (German: storeroom or warehouse) is a type of beer that is conditioned at low temperatures, normally in cold storage at the brewery, before being delivered to the consumer.It may be pale, golden, amber, or dark.ager beer uses a process of cool fermentation, followed by maturation in cold storage. The German word Lager means storeroom or warehouse. The yeast generally used with lager brewing is Saccharomyces pastorianus. It is a close relative of the Saccharomyces cerevisiae yeast used for warm fermented ales.