The simplest explanation for the difference between lagers and ales is that they use different yeasts during fermentation. Lagers are made with lager yeast and ales are made with brewer's yeast. The basic difference between these two main classifications of beer is the way in which they are fermented. Beers are fermented with high-fermented yeast at warm temperatures (60° to 70° F) and lagers are fermented with lower-fermented yeast at cold temperatures (35° to 50° F).
Because of their warm fermentations, beers can generally ferment and age in a relatively short period of time (3 to 5 weeks). On the other hand, lagers take much longer to ferment (up to 6 to 8 weeks) because they are cold fermented. Beer is made by allowing yeasts to colonize a sugary, grain-based liquid (called wort), where they convert sugar into alcohol and carbon dioxide, the basic components of beer. The fundamental difference between beer and lager beer is that different yeast species perform this conversion at different temperatures.
It's a subtle difference in theory, but profound in practice. The reason why craft breweries almost exclusively produce beers is because the time and storage requirements to make quality beers consume much more money than beers, which can be fermented, hopped and canned in just a few weeks. However, in today's craft beer market, ale-type beers tend to be more common among craft brewers because brewer's yeast can produce beer in as little as 7 days, making it more convenient for small breweries that don't have space in the fermenter to produce beer on a regular basis. After fermentation, lagers traditionally, but not necessarily, undergo a period of rest of up to several months in contact with their yeast.
What this means for beer, in general, is that lagers have fewer fruity, spicy and complex flavors from yeast metabolism, and more of a simple, clean flavor that comes from the grain. They mass-produced the diluted, unflavored “lager” beer that a true craft beer lover wouldn't stay dead drinking. Lagers are still popular, but the revival of beer has broadened the palates of many beer drinkers and altered the course of beer history in the most positive way. This contrasts markedly with the clean, crisp, rounded flavors and aromas of beer fermented with lager yeast.
When the cells clump together, they sink to the bottom of the vat, instead of floating, which earned lager yeast the name lower fermentation yeast. Well, despite what the lazy marketing of macrobreweries might lead you to believe, a lager beer is a broad category of many different styles. Lagers were originally fermented in caves in the cold months and were drunk in spring, when the weather warmed up and the yeast finished its work. Unless you consider a Doppelbock with 9.5% alcohol to be a “low alcohol” beer, lagers can sometimes be the strongest beers offered on a menu.
There's a lot of biochemistry needed to fully understand it, but suffice it to say that lager yeast traditionally requires a little more time to prepare than beers. Contrary to a strange popular belief, the color or clarity of a beer has nothing to do with its classification as an ale or lager. Lager is a noun (which translates from German as “storage”), but it is also a verb that refers to the cold storage of beer while it is fermented. Today, there are more than 6,000 breweries in operation, creating beers, lagers and combinations of both that have brought beauty and art back to brewing.