Perhaps the question we should ask is: What is an ale and what is a lager?
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 species, fermentation temperature, aging temperature & duration, and fruit character.
Yeast
This is the only real physical difference between ales and lagers. 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. Reasoning for distinguishing them as bottom or top-fermenting seems to correlate with the amount of kräusen found at the top of the fermenter.
FYI: Kräusen is the fluffy stuff that develops at the top of the fermenter that is made up of yeast and proteins and other materials frothed up as a result of yeast metabolism.
Fermentation Temperature
In general, ale yeast is usually slated to ferment 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 and development of a large cap of kräusen. Lager yeast typically ferment at cooler temperatures (48°F to 58°F, on average). Because of this cooler temperature, fermentation is less violent, takes much longer, and there is less kräusen.
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 cool fermentation, lagering at about 33°F helps to further clarify the beer and allows the flavors to round out. Lagers can take many months to complete, from brew-day to packaging. Depending on the beer, lagering alone can last upwards to 6 months to a year, although 3 months is average.
Fruit Character
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, generally higher temperatures, and short aging. This is acceptable and desirable. Lagers on the other hand, are clean and show no fruit character. The slow fermentation caused by low temperature and long lagering times result in a malt and hops focused beer.
There will be more explanation further below, but the same can be true with beers fermented cold with ale yeast strains, which will produce very little esters. Interestingly enough, lager strains used to ferment beers at ale temperatures will indeed produce more fruit esters than they would usually do if fermented cold.
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 to almost 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 producing a foam head at least 3 fingers high. Just try it.
Curveballs
This is probably the most important part of this article. We have hybrid beers out there to throw us a off-course and shows off how far we have come in our understanding of what used to be a clear line between ale and lager. These hybrid beers are a blend of ale and lager, usually using lager yeasts fermented at the lower end of ale temperatures and/or often involve lagering for a period of time regardless of yeast strain or species. Examples of these hybrids include Cream Ale, California Common, Kölsch, and Altbier.
California Common and Cream Ale use an adapted lager strain to ferment at the low-end of ale temperatures (too high for some lager strains) and results in clean lager characteristics. This is historically true for Cream Ale, but is currently not that common. Kölsch and Altbier are examples of an ale yeast strains fermenting at the high-end of lager temperatures (too low for some ale stains) used to produce lager characteristics. They also go through a period of cold conditioning although the duration varies by style. Those are some examples of styles which create clean, lager-like beers using non-traditional methods.
A word on lagering: It is not uncommon for a beer fermented with ale yeast to undergo a cold-conditioning phase or for a "bottom-fermented" beer to be aged at room temperature. The resulting beer may differ from the traditional expectations, but could still be enjoyable if done correctly.
And there you have it!
To recap, ale yeast (Saccharomyces Cerevisiae) and lager yeast (Saccharomyces Pastorianus) is the only physical difference between them. The flavor 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. There are exceptions with hybrid's using lager strains at low ale temperatures or ale strains at high lager temperatures.
Hope you learned a thing or two. Thanks for reading!
Cheers,
Gilbert "Charlie" Perez, Certified Cicerone®