The winery may then keep the wine so that there can be additional clarification and, in some wines, to give it a more complex flavor.
Flavor can come from wood (specifically, from the chemicals that make up the wood that are taken up into the wine). When wood aging is used, wines are stored, most commonly, in oakwood barrels. It is considered by many that French oak barrels give the best flavor and that they must be replaced after several years of use. American oak is used by some producers and you can usually taste the difference. Other producers will buy older, used French oak barrels and create wines that some feel are inferior (but they're most likely less expensive).
Some wines may never see anything but stainless steel and the glass that they are bottled in. In any event, using oak barrels puts an 'oakiness' characteristic in wine. The wine may be barrel aged for several months to several years.
A brand new low-end American oak barrel costs around US$200.
A brand new high-end French oak barrel costs around US$700.