Seasoned beer drinkers can often tell the difference between ales and lagers. Ales are usually described as "robusty, hearty and fruity". Lagers are characteristically "smooth, elegant, crisp, fruity, and clean". The fermentation process when the beer is brewed is different for lagers and ales.
Ales are brewed with top-fermenting (actually ferments throughout the wort) yeast which allows for rapid fermentation at warmer temperatures; Lagers are brewed with bottom-fermenting yeast which ferments more slowly and at colder temperatures.
Lager beer is made with bottom yeast, so called because it works at the bottom of the vat. Traditionally bottom yeast will ferment at cold temperatures less than 10 deg C. Now fermentation takes place at 12 to 18 deg C. This cold or deep fermentation allows the malt and hops to assert their fine flavours.
On the other hand, ales are fermented at temperatures from 15 to 25 deg C. Ales are matured for shorter periods and at warmer temperatures.
Ales are usually described as "robusty, hearty and fruity". Lagers are characteristically "smooth, elegant, crisp, fruity, and clean".
edit Sub types
Ales include everything with ale in the name (pale ale, amber ale, etc.), porters, stouts, Belgian specialty beers, wheat beers and many German specialty beers. They generally have a more robust taste, are more complex and are best consumed cool (45F or a bit warmer) rather than cold.
Lagers include pilseners, bocks and dopplebocks, Maerzens/Oktoberfests, Dortmunders and a few other styles found mostly in Germany. They are best consumed at a cooler temperature than ales, although anything served at less than 38F will lose most of its flavor.
edit Ale vs Lager Videos
Here are some good videos explaining the difference between ale and lager beer
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