Yeast is a one-celled fungus that converts sugar and starch into carbon dioxide bubbles and alcohol. This has made it a useful ally in the production of bread, beer, and wine.
There are many varieties of yeast. Bread is made with baker's yeast, which creates lots of bubbles that become trapped in the dough, making the bread rise so it's light and airy when baked. A small amount of alcohol is also produced, but this burns off as the bread bakes.
Beer yeast and wine yeast are used to convert sugar into alcohol and, in the case of beer and champagne, bubbles.
You should never eat raw active yeast, since it will continue to grow in your intestine and rob your body of valuable nutrients. But once deactivated through pasteurization, yeast is a good source of nutrients. Brewer's yeast and nutritional yeast, for example, are sold as nutritional supplements, and Australians are fond of yeast extracts--like Vegemite, Marmite, and Promite--which they spread like peanut butter on bread.