Yeasts are living organisms, a uni-cellular fungus, and have a rare ability: to live with or without oxygen.
In the presence of air, they multiply. In the absence of oxygen, they ferment sugars into alcohol. That's what makes beer brewing possible.
Beer styles are distinguished by the two main types of yeast used to ferment the wort (the liquid made from water and malt, flavored with hops), to make ales or lagers.
Ale yeast is said to be 'top-fermenting' since the yeast cells tend to accumulate ('flocculate') at the top. Lager yeast migrates to the bottom of the tank during fermentation and so is called 'bottom-fermenting'.
In the case of ale yeast, some interaction with oxygen takes place during fermentation. Ale yeast ferments quicker - a few days to two weeks - and at higher temperatures (around 21ºC/70ºF), though this can vary from as low as 10ºC/50ºF to as high as 25ºC/77ºF. Brews made from it also tend to store longer and have a higher alcohol content.
Lagers ferment more slowly (up to a month) and at lower temperatures, sometimes as low as near freezing. As a result, historically, lagers were often brewed in the winter and consumed later. Temperature ranges vary, though, and can easily be in the higher 45ºF-59ºF/7ºC-15ºC range. With modern refrigeration technology came the option of having lager year-round.
There's also a third type, used originally almost exclusively in Belgium: Lambic yeast. The name derives from the West Flanders area in Belgium where the yeasts grow wild. Today, as a result of importing, it's used in many parts of the world.
These helpful creatures transform malt sugar (maltose) into alcohol (ethanol) and carbon dioxide - the basic fermentation process. But, as its bread-making cousins do, yeast also adds distinctive flavors to the brew.
Many ale yeasts have a full-bodied, fruity aroma and taste. Others are more nutty or mineral tasting, suitable for stouts or Belgian ales and other strong brews.
One variety is used in Weizenbier, or wheat beer. The primary ingredient in beer is malt, from the cereal grain, barley. The name 'wheat beer' comes not from the grain used, but from the yeast used to ferment it. Wheat beer yeast goes into this ale-style brew, where it helps produce a fruity, intense character.
Lager yeasts are often smoother and dryer, with the taste of cloves, vanilla or a wide variety of other hints. Pilsner, for example, is a type originating in the Czech Republic in the town from which the brew gets its name.
Once, it formed the basis of 90% of the lagers consumed around the world. But with the growth of micro-breweries and the expansion of variety in the U.S. and elsewhere, lagers now come as Dortmunders, Märzens, Bocks and other styles as well.
Though carbon dioxide and ethanol are the two primary products of fermentation, yeast produces secondary products as well. The different types can add tastes or aromas as varied as sweet corn to green apple to butterscotch. Unfortunately, they can also produce sulfur, or phenolics, which have a medicinal taste.
Control of the added flavor is as much an art as the control of fermentation is a science.