Most often, corked wine results from a bad cork, though it can also come from something happening during the aging or bottling process, or even during the storage of the bottle. Corks can get tainted by pesticides, airborne fungi, or bacteria while they are still on the tree. Corks also get contaminated with a chemical called TCA (2,4,6-Trichloroanisole). TCA is also detected in the wood of wine barrels and in the hoses, valves, funnels, and other objects involved in winemaking. TCA is harmless to humans, but it interferes with the odor and taste of wine to the point where it is unpleasant.
If your wine is corked, you should return it to the store or refuse it at the restaurant.
(Image: Kathryn Hill)