Yeasts that ferment at relatively warmer temperatures, usually between 15-24 degrees Celsius, form a layer of foam on the surface of the fermenting beer, which is why they are referred to as top-fermenting yeasts.
Yeasts that ferment at considerably lower temperatures, below 10 degrees Celsius, have the ability of to process a chemical compound known as raffinose, a complex sugar created during fermentation. These yeasts collect at the bottom of the fermenting beer and are therefore referred to as bottom-fermenting yeast. The majority of beer in production today is fermented in this way and is called lager.
Two other types of beer styles include beer of spontaneous fermentation and beers of mixed origin. Beers of spontaneous fermentation are mainly produced in Belgium using wild strains of yeast. These types of beers are often referred to as Lambic.
Beers of mixed origin include Altbier and Kölsch.