Yeast is a living, microscopic, single-cell organism that, as it grows, converts its food (through a process known as fermentation) into alcohol and carbon dioxide. This trait is what endears yeast to wine makers, brew masters and bread bakers.
In the making of wine and beer, the manufacture of alcohol by baker's yeast is desired and necessary for the final product, and carbon dioxide is what makes beer and champagne effervescent.
The art of bread-making needs the carbon dioxide produced by yeast in order for certain doughs to rise and expand, becoming lighter and fluffier in consistency. To multiply and grow, all yeast needs is the right environment, which includes moisture, food (in the form of sugar or starch) and a warm, nurturing temperature (21° to 30°C / 70° to 85°F is best). As the yeast consumes the sugar and starch in the dough it produces small pockets of carbon dioxide. As the dough bakes, the carbon dioxide expands, and gives the bread its airy texture.
Types of Yeast 
The three yeast types commercially available are baker's yeast, brewer's yeast and nutritional yeast. Baker's yeast, as the name implies, is the type used as a leavening agent.
Baker's Yeast 
Baker's yeast is categorised into three basic types — active dry yeast, compressed fresh yeast and yeast starters.
Active dry yeast is in the form of tiny, dehydrated granules. The yeast cells are alive but dormant because of the lack of moisture. When mixed with a warm liquid (40° to 46°C / 105° to 115°F), the cells once again become active. Active dry yeast is available in two forms, regular and quick-rising , of which the latter takes about half as long to leaven bread. They may be used interchangeably (with adjustments in rising time) and both are available in 1/4-ounce envelopes. Regular active dry yeast may also be purchased in 4-ounce jars or in bulk in some health-food stores. It should be stored in a cool, dry place, but can also be refrigerated or frozen. It should always be at room temperature before being dissolved in liquid. Properly stored, it's reliable when used by the expiration date, which should be stamped on the envelope or jar label. One package of dry yeast is equal to 1 scant tablespoon dry yeast or 1 cake of compressed fresh yeast.
Compressed fresh yeast, which comes in tiny, square cakes of about 50g, is moist and extremely perishable. It must be refrigerated and used within a week or two, or by the date indicated on the package. It can be frozen, but should be defrosted at room temperature and used immediately. One cake of fresh yeast can be substituted for one 1/4-oz envelope of dry yeast. In the United States, the use of compressed fresh yeast has been mostly replaced by the more longer-lasting active dry yeast.
All baker's yeast should be given a test called proofing to make sure it's still alive. To proof yeast, dissolve it in warm water and add a pinch of sugar. Set the mixture aside in a warm place for 5 to 10 minutes. If it begins to swell and foam, the yeast is alive, active and capable of leavening bread.
Brewer's Yeast 
Brewer's yeasts are special non-leavening yeasts used in beer making. Because it's a rich source of B vitamins, brewer's yeast is also used as a food supplement. It's available in health-food stores. Brewer's yeasts are also marketed in speciality beer making equipment shops, with different strains used for different beers.
Nutritional Yeast 
Nutritional yeast, similar to brewer's yeast, is a nutritional supplement popular with vegans and the health conscious, who use it as an ingredient in recipes or simply as a condiment. It is a deactivated yeast, usually Saccharomyces cerevisiae.
It is a source of protein and vitamins, especially the B-complex vitamins. It is also naturally low in fat and sodium. Some brands of nutritional yeast, though not all, are fortified with vitamin B12. The vitamin B12 is produced separately from bacteria and then added to the yeast.
Nutritional yeast has a strong flavour that is described as nutty, cheesy, or creamy, which makes it popular as an ingredient in cheese substitutes. It is often used by vegans in place of parmesan cheese. Another popular use is as a topping for popcorn. Some movie theatres offer it along with salt or cayenne pepper as a popcorn condiment.
It is commercially available in the form of flakes,photo or as a yellow powder similar in texture to cornmeal, and can be found in the bulk aisle of most natural food stores. In Australia it is sometimes sold as "savoury yeast flakes." In Canada it has been known to be called "good tasting yeast".
Nutritional yeast is different from yeast extract, which has a very strong flavour and comes in the form of a dark brown paste. Other species of yeast are used for making beer, wine, and bread.
Wild yeast spores are constantly floating in the air and landing on uncovered foods and liquids. No one's sure when these wild spores first interacted with foods but it's known that the Egyptians used yeast as a leavening agent more than 5,000 years ago. Wine and other fermented beverages were made for millennia before that. Today, scientists have been able to isolate and identify the various yeasts that are best for wine making, beer making and baking.