A 'Corked' wine is a wine that has been bottled with a cork that is contaminated with TCA (2,4,6-Trichloroanisole). TCA contamination usually comes from corks but can also come from barrels, other cooperage or even, apparently, from wood within the cellar including walls or beams. The term 'corked wine' is applied to all wines with TCA contamination because corks are the souce of most of the problems. The wine industry estimates that as many as 3% to 7% of all wines have TCA contamination at levels that can be detected by consumers. Because most people are not trained to recognize the smell and taste of TCA, only a very small fraction of these bad bottles are ever returned to stores or sent back at a restaurant.
Even a very tiny amount of TCA in a wine can ruin it. Most people become aware of TCA in quatities as small as 5 parts per trillion and some individuals are even more sensitive. When TCA is present in quantities high enough to be evident to a person, it comes across as 'musty' aromas and flavors. Even when TCA is not evident in the smell or taste of a wine, very small quantities can subdue the aromas and flavors of fruit that the wine would ordinarily exhibit.
TCA does not pose a health risk (at least in the levels found in wines). It just imparts the aromas and flavors that are objectionable when found in sufficient quantity. Many wines have levels of TCA that are below the threshold of perception. Wine is not the only place you can find TCA. It is also found in some municipal water supplies as well as in some teas.
A great deal of work continues in the cork industry as well as at wineries to develop methods to eliminate corked wine. So far, no completely reliable method has been found.
There are other causes of bad bottles of wine, but TCA contamination is the primary fault you will find in otherwise well-stored bottles. Other faults can include wines that are oxidized, lightstruck or have undergone unplanned secondary fermentation.
If you get a 'corked' wine, you should return it to the store from which it was purchased or refuse it at the restaurant. Most wineries completely stand behind their wines and will work to ensure customer satisfaction. Do make sure that you check the wine when it is opened and before it is poured around the table. Wineries and stores are less likely to accept the return of an empty or nearly empty bottle with your claim that it was bad. The tasting ritual of a freshly opened bottle of wine developed over the years to allow the host to check and make sure that bad (corked) wine was not poured for guests.