Wikipedia says that
"Yeast, most commonly Saccharomyces cerevisiae, is used in baking as a leavening agent, where it converts the fermentable sugars present in the dough into carbon dioxide. This causes the dough to expand or rise as the carbon dioxide forms pockets or bubbles. When the dough is baked it "sets" and the pockets remain, giving the baked product a soft and spongy texture. The use of potatoes, water from potato boiling, eggs, or sugar in a bread dough accelerates the growth of yeasts. Salt and fats such as butter slow down yeast growth. The majority of the yeast used in baking is of the same species common in alcoholic fermentation. Additionally, Saccharomyces exiguus (also known as S. minor) is a wild yeast found on plants, fruits, and grains that is occasionally used for baking. Sugar and vinegar are the best conditions for yeast to ferment. In bread making the yeast respires aerobically at first producing carbon dioxide and water. When the oxygen is used up anaerobic respiration is used producing ethanol as a waste product; however, this is evaporated during the baking process."
Fresh or compressed yeast comes in a beige coloured cake. It works faster and longer than dried yeast but is perishable it has a four week life if kept in the fridge but will last for four months when frozen. It is mostly used by commercial bakers. Available in some supermarkets and from bakers.
Dried yeast comes in granules that must be mixed with warm water. It lasts far longer than fresh yeast and is in common use at home. Available in most supermarkets.