This mixing process is necessary to expose all the grape juice to the skins for a chosen period. That's right, red grapes actually produce white juice that only turns red after prolonged exposure to its skins. Keeping the juice off the skins is how Zinfandel, a red grape, can be made into a white wine. Though tannin is a key element to most red wines, it is undesirable in whites, which is why white grapes are immediately pressed. Most red wines from Northern California are barrel aged to some extent.
Wine barrels come in only a few shapes and sizes and are made exclusively from oak.
Some barrels, however, are made with American oak and others with French. Most wineries use a combination of both. Differences are subtle, but conspicuous. Chardonnay, being a white wine, and thus lighter in flavor, is where you will find the oak flavor most detectable. Think of trees the next time you sip a Chardonnay that has been aged in oak and see what it gets you.