Oak barrels have been used to store and transport wine since the Roman Empire. Over time, wine makers began to consider the barrel a part of the process as they noticed that the oak wood changes the wine's color and enhances its flavor and texture.
When in an oak barrel, mainly alcohol and water of the wine evaporate, concentrating the flavor and aroma compounds. Oxygen enriches the tannins of the wine, thus decreasing its bitterness.
Most winemakers use American or French oak, which is distinguished for the sweet, fruity, vanilla-like flavor it creates in the wine.