Virtually every single beer you’ve ever enjoyed can be broken into one of two main types: ale or lager. The determining factor between the two? Yeast.
Yeast is a single-celled microorganism responsible for fermenting many of our favorite adult beverages: beer, wine, cider, mead, and even in the beginning stages of distilled spirits like whiskey (not to mention bread, cheese, aged meats, etc.)—yeast is a truly wonderful thing. Yeast works by consuming sugar and turning those sugars into carbon dioxide (the fizzy bubbles in beer), alcohol, and other chemical compounds.
Generally speaking, ale yeasts prefer warmer temperatures and are considered “top fermenting” based on the location of the fermentation activity in a fermentation tank. I.e. they ferment while sitting on top of the beer in a tank. Lager yeasts prefer cooler temperatures, ferment more slowly, and are considered “bottom fermenting.” The word lager comes from the German word which means “to store” – lagers were first stored in cool caves to mature.
Lagers are the most widely drank beers in the world, though they are younger historically than their ale counterparts. Yeast wasn’t even identified as the actual magic behind beer and other fermented beverages until the 1800’s. Lager yeast was isolated in 1883, where wild ale yeasts have been used (unknowingly at first) since the dawn of modern human civilization thousands of years ago. Because lager yeasts were some of the first to be isolated in their pure form without wild contamination, lager beers originally had longer shelf lives. Some believe this led to their wide distribution over ales, which did not age well at the time. The advent of refrigeration allowed for the year-round making of lager beer. Nowadays there are hundreds of known ale and lager yeast strains, and brewers can select them just like they’d select a certain type of grain or hop.