The role of oak in wine
Oak is used to help either age or ferment wine. Using oak will affect colour, flavour, tannin structure and mouth feel. Wines can spend anywhere from a few months up to a few years in oak (as with Rioja).
Aromas of vanilla, wood, or cedar can be imparted through the process. Oak allows low level exposure to oxygen, which in turn evaporates some of the wine and changes the flavour. Typically this softens tannins and increases aromas.
With whites, oak makes the mouthfeel silkier, and it adds aromas such as butter, cream, caramel, smoke, and vanilla.
There are two main types of oak: American and French. French imparts subtle and silky tannins, while American oak can be more astringent, with aromas of coconut, sweet spices and dill.