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 a short, warm fermentation 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 Düsseldorf.

• 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.

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