The differences tend to be based on tradition more than anything
     inherent to either style. The major traditional differences are a
     result of the varying lengths of fermentation and temperature used
     for the two beer types. They can also vary in style and degree of
     hopping and in the types of malt used, but these differences are very
     arbitrary and exceptions abound.

     Ales generally undergo short, warm fermentations and are intended to
     be consumed soon after completion. The result of relatively warm
     fermentation is that a lot of by-products of yeast metabolism besides
     alcohol and CO2 get left in the beer. These usually manifest
     themselves as "fruity" or "buttery" flavors which vary in degree and
     flavor with the strain of yeast used and the temperature and duration
     of fermentation. Accordingly, ales exhibit their most complex flavors
     when served at warm temperatures, around 50-60F (10-15C).

     The trick with lager yeast is that they can survive, metabolize, and
     reproduce at lower temperatures. Lager yeast can assimilate compounds
     which ale yeast cannot, fewer by-products are made, and the stuff
     that does get made drops out during lagering. The result is a very
     clean, sparkling beer. Lagers are best served at slightly cooler
     temperatures than ales, 40-50F (5-10C).

     Of course there are notable exceptions:

     California Common
          The best known example is "Steam Beer" which is a trademark of
          the Anchor Brewing Co. It employs lager yeast fermented at ale
          temperatures which gives it some fruitiness usually associated
          with ales.

     Koelsch and Alt
          Ales that undergo a cold secondary fermentation and storage
          period resulting in only a hint of ale-like fruityness. Koelsch
          is usually associated with the city of Cologne, Germany while
          Alt is indigenous to Duesseldorf.

     Cream Ale
          Alternately, an ale fermented at lager temps or vice-versa. It
          has also been made by blending a conventional ale with a
          conventional lager after fermentation. Most examples are only
          slightly more interesting than mega-brews; a touch more body, a
          touch more fermentation flavor.