Yeast primarily survives by eating simple sugars like glucose and fructose. There are many types of yeast, some of which can eat different sugars, including disaccharides, which are slightly more complex than simple sugars.
Yeast is a one-cell fungi organism. It is widely used in baking wheat-based foods like bread, but is also used to make beer, wine and certain cheeses. When a yeast organism consumes sugar, the byproducts include carbon dioxide and alcohols. Many alcoholic drinks are created from this process, which is called fermentation. Wine is simply the fermentation of a fruit, typically grapes. The produced carbon dioxide can make baking goods "rise," which is why yeast is used in breads.