Oak barrels are used for maturing the wines in the cellar before bottling. The juice can be fermented and macerated directly in the barrels (standing up and one end taken off) which some believe gives a better integration of the wood. More commonly the wine is racked to the oak barrel after finished fermentation and left to mature there from 6 to 36 months. The oak allows the wine to breathe via a minuscule exchange of oxygen between wood and wine. This rounds off the texture of the wine and it also helps stabilise the colour pigments in red wines. Finally, the oak also contributes with tannins and flavour compounds like vanilla, smoke and spices to the wine. The barrel adds more aromas and tannins the first years and becomes a more neutral vessel for maturing with age.
There a many kinds, sizes and qualities of oak barrels and equally many different beliefs and styles on how to use them.