Fermentation is the heart of wine. Grapes are harvested and crushed so that the juice — the must — is released. Yeast cells, either naturally present or added, begin to consume the sugar in the must. The process produces alcohol and carbon dioxide and typically lasts days to weeks depending on the style.
When all sugar is consumed, or when the winemaker stops fermentation early, wine is what remains. Red wine ferments with the grape skins left in the tank, which gives colour and tannin. White wine is pressed first, and only the clear juice ferments. This simple difference explains much of what distinguishes red from white.