What does fermentation do to grape juice?

Fermentation is basically when yeast grows on the SKIN of grapes. When the skin is broken down, the yeast feeds on the glucose ( which is in the Grape Juice ) which turns it into ethanol ( alcohol ) BUT and the SAME time it makes carbon dioxide. This is used in wine making, so it doesn't do anything to the grape juice, but it helps make it after the ethanol is taken out and the rest is purified.