A lager is a beer that is produced with a bottom fermenting yeast strain. In general, lagers are fermented at cooler temperatures (40 - 50ºF) and for a longer period of time (4-6 weeks) than ales.
Most bottom fermenting yeast strains do not produce flavor or aroma altering compounds commonly found in ales, so the characteristics of lagers are usually straightforward, and come mostly from malt and hops. Like ales, lagers can range dramatically in color from straw-gold to black, and can be found with a huge variance in strength, from 0.5% ABV to more than 12% ABV.