Wine has been made for centuries from just a two simple ingredients: yeast and grape juice. Actually, just about any fruit juice can be used, but by far the majority of all wine is made from the juice of the grape.
Yeast is the magical ingredient that turns grape juice into wine. Interestingly enough, there is actually wild yeast spores in the air and all that is really needed to make wine is an open container of grape juice and time. The result however, would probably not be the most palatable of beverages.
There are numerous strains of yeasts and the types used to make wine have been cultured just for this purpose. Well anyway, yeast is a living organism that feeds off of sugars in the grape juice in a process called fermentation.
During fermentation, yeast spores will reproduce exponentially until all of the fermentable sugars have been consumed. During this fermentation process, the sugars are converted into alcohol and carbon dioxide.