The short answer is... nothing! Well, that is if we consider only the end product, which is still beer. But if we dissect this, there are only a few differences. The main few are yeast, fermentation temperature, aging temperature & duration, and fruit character.
There are two types of yeast that brewers choose from. Ale yeast (also known as top-fermenting yeast) is called Saccharomyces Cerevisiae. This species of yeast has many different strains. Lager yeast (also known as bottom-fermenting yeast) is called Saccharomyces Pastorianus. As with ale yeast, this species also has many different strains. They both act the same in terms of basic fermentation metabolism and end product.
In general, ale yeast ferments at higher temperatures (65°F to 70°F, on average with some strains reaching even higher) and thus results and a vigorous, relatively quick fermentation. Lager yeast ferments at cooler temperatures (48°F to 58°F, on average). Because of this cooler temperature, fermentation is less violent and takes much longer.
Aging Temperature & Duration
Ales are generally not aged for very long and are usually cold crashed (significant drop in temperature to about +/-40°F) after fermentation to allow the yeast and proteins to drop out. Dry-hopping (adding hops into the fermenter after fermentation is complete) is done at this time. Typically, an average ale is ready in as little as two to four weeks.
Lagers ferment at cooler temperatures and therefore take longer to complete. Fermentation can last a few weeks and up to a couple months in some cases. Once fermentation has finished, "lagering" can start. The word "lager" is the verb "to store" or put away in German Combined with lagering temperatures down to about 33°F helps to further clarify the beer and allows the flavors to round out. Lagers can take many months to complete, from brewday to packaging. Depending on the beer, lagering alone can last upwards to 6 months to a year, although 3 months is average.
The aforementioned items are production-based distinctions. All of those aspects result in the one piece of evidence we can detect in both flavor and aroma: Fruit character. Ale yeast produce many fruity esters due to the quick fermentation and short aging. This is acceptable and desirable. Lagers on the other hand, are clean and show no fruit character. The slow fermentation and long lagering times result in a malt and hops focused beer.
Bonus: Myths About Ales and Lagers
"Lagers have lower alcohol than ales." ...FALSE!
Being an ale does not necessarily indicate a higher ABV. True, the high-alcohol beers are usually an ale of some kind, there are also lagers available that can reach ABV's above 10%. Eisbock is one example.
"Ales are dark." ...FALSE!
Cream Ales and American Blonde Ales are quite pale in color.
"Lagers are always light in color." ...FALSE!
Doppelbock, Dunkle, American Dark Lager, and Baltic Porter are examples of lager beers that are dark brown and can be even black in color.
"Lagers have more carbonation." ...FALSE!
Have you ever popped opened a bottle of Duvel? That's a Belgian Golden Strong Ale. Try to pour it without a cap of foam at least 3 fingers high. Just try it.
We have hybrid beers out there to throw us a off course. These hybrid beers are a blend of ale and lager, usually using lager yeasts fermented at the lower end of ale temperatures and often involve lagering for a period of time. Examples include Cream Ale, California Common, Kölsch, and Altbier.
There you have it!
To recap, the differences between lagers and ales are small, but noticeable. Lagers are clean, little no no yeast character with and no fruity esters, fermented a cool temperatures, and are submitted to extended lagering. Ales tend to have fruit character, fermented at higher temperatures, and not aged as long by comparison.
Hope you learned a thing or two. Thanks for reading!
Gilbert "Charlie" Perez, Certified Cicerone®