During anaerobic respiration, yeast cells produce ethanol (alcohol) and carbon dioxide. The carbon dioxide is what makes bread rise during the baking process.
Yeast are microorganisms that are classed with fungi, and they use anaerobic respiration, which does not require oxygen. The yeast consume glucose, the sugar found in plant life. Yeast are used to make bread rise, which is why bread must be proofed before it can be baked. The proofing process, where the bread is left in a warm, moist area for an extended period of time, allows the yeast to multiply as well as consume sugars, converting them to carbon dioxide.
Yeast are also used to make a number of other foods, including wine, beer, yogurt and vinegar. The process of anaerobic respiration is called fermentation when referring to foods. In wine or beer, the yeast convert sugars in the fruit into alcohol over time. In fact, the carbon dioxide they produce creates the bubbles in some drinks. As vinegar is produced from alcohol, it is another byproduct of yeast. While yeast is naturally found everywhere, including on fruits, yeast used in these processes are specifically developed for these purposes because they produce the same results time after time in the fermentation process.