Brewers yeast is categorized into two main varieties. They are classified as ale yeast (top-fermenting type, Saccharomyces cerevisiae) or lager yeast (bottom-fermenting type, Saccharomyces uvarum). These two varieties are further broken down into categories of specific strains. There are hundreds of strains of both ale and lager yeasts.
Ale yeast is a variety of yeast that is best used at temperatures ranging from 65-75°F with certain ale strains performing well and indeed optimally as low as 55 °F. Lower temperatures tend to inhibit fermentation; some strains will not ferment below 50 °F. Some strains of ale yeast exhibit a tendency to flocculate at the surface of the fermenting beer during the first few days of fermentation, hence the term "top-fermenting."
Lager yeast is a variety of yeast that is best used at temperatures ranging from 45-55 °F with certain lager strains performing well at temperatures as low as 32 °F. The desired "smoothness" of lager beers is best achieved from fermentations carried out at the lower end of this temperature range for anywhere from three weeks to many months. All strains of lager yeast will flocculate and then settle to the bottom of the fermenting vessel, hence the term "bottom fermenting."