There are two main strains of yeast that have been cultivated specifically for beer brewing: ale yeast and lager yeast. It's possible to use the wild yeast living naturally in our environments to brew beer (as for some Belgian styles), but wild yeast is somewhat unpredictable and can result in some very funky brews. For home brewing purposes, it's usually best to stick with the commercially-made yeast.
Ale yeast ferments best at anywhere from room temperature down to about 55°. It's called "top fermenting" because you can actually see the yeast gather at the top of the beer in the first few days of fermenting (though it will eventually settle into a sediment at the bottom).
Lager yeasts ferment best at slightly cooler temperatures, anywhere from 32° to 55°. They also tend to gather at the bottom of the fermenting tank, making them "bottom fermenting."
Choosing one kind of yeast over the other depends on the specific style of beer you're trying to brew and the flavor you're aiming for. Although other factors and ingredients influence flavors, ales tend to have complex, rather fruity flavors while lagers usually have a clean, smooth taste.
Both ale and lager yeasts feed on the sugars in the malt to produce alcohol, carbon dioxide, and other flavors. This fermentation process goes on for between 5 and 14 days, after which the yeast has more or less finished converting all the existing sugars. Additional sugar can be added during bottling to encourage more carbonation in the final brew.
Have you done any homebrewing? What has been your experience with ale and lager yeasts?
(Image: Emma Christensen for the Kitchn)