Fermentation is a natural process that converts the sugar in any sweet material (in our case, grapes) to alcohol and carbon dioxide gas. Once the grape skins are broken, the yeast organisms on the skins go to work on the sugar content, which is about 30% of the pulp.
In principle, the yeast will continue working until all the sugar in the grapes is converted to alcohol, or until the alcohol level reaches about 15%.
A dry wine is completely fermented, with no sugar remaining. Fermentation can be stopped early to obtain sweeter wines. Methods of stopping the fermentation include adding alcohol to reach 15% or adding sulphur, both of which stop the yeast action; the wine can also be put through a very fine filter to remove the yeast.
Dry wine remains in the vat until all the sugar is converted to alcohol.
Sweet wine is taken from the vat before the fermentation is complete, and the fermentation is stopped.
The wine is taken early from the vat and bottled. The fermentation continues in the bottles until it is completely finished, and the carbon dioxide remains.