Yeast are microscopic fungi; they are living organisms. As such, they have optimal temperatures at which they function, and that varies depending on the species of yeast. Humans take advantage of one of the chemical processes that yeast perform: fermentation breaks down sugars into alcohol and carbon dioxide. This is used to produce bread (the gas bubbles of carbon dioxide make the bread rise), and alcohol products such as beer and wine.
Best temperatures for growth can be as low as from 28-68 degrees F., to 82-112 degrees F. For yeast used for bread baking, the optimal temperature range is 75-85 F. Different beers have different fermentation temperatures, generally in the 50s or 60s. Wines vary as well, with fermentation temperatures warmer for reds (about 80 F.) than for whites (about 55 F.)
Fermentation itself generates heat, so it can be difficult to keep temperatures from rising to a level that will kill the yeast.