Decanting, or pouring wine from the bottle into a carafe or decanter, is done primarily for the purpose of separating the sediment from the wine. While this is not a necessity for many wines, it is most important with older red wines, especially Ports, in which sediment can be a problem.
Tannin and other particles solidify in the wine. This may take up to eight years. They should be removed for two reasons – they can taste bitter, and they do not very look appetizing floating around in the wine. To remove sediment you should keep the bottle in an upright position for a couple of days before decanting to allow it to settle to the bottom of the bottle. Then pour the wine slowly and carefully into the decanter all the while watching the neck of the bottle for the wine to become cloudy indicating that the reminder of the wine contains unacceptable amounts of sediment.
Another benefit of decanting wine is that it is aerated in the process. Many times people say that the wine needs to "breathe" when they are actually referring to aerating it. This is especially beneficial for young red wines like a California Cabernet Sauvignon, and should be done quickly to expose the wine to as much air as possible. Simply opening the bottle and allowing it to sit for a while is definitely insufficient for aeration because the tiny space at the neck of the bottle is far too small to allow much air to get to the wine.
It is not necessary to have an expensive decanter as any large glass will do, but as I have said in many of my articles, for me, ambience is important. A nice crystal wineglass and a pretty decanter enhance the experience. If you choose to aerate the wine in glasses, pour it into a large glass and allow it to stand for at least ten minutes before drinking.
Certain wines benefit more from decanting than others. Among them are young, tannic red wines like Cabernet Sauvignons, Bordeaux, and many Italian wines because their tannins soften and the wine becomes less harsh. The younger and more tannic the wine, the longer the time required to allow it to breathe. Young Vintage Port especially benefits from several hours of aeration due to the fact that they are so brutally tannic. Older Vintage Ports need decanting because of the excess sediment. They should stand upright for several days before decanting. For most, though, one hour is a sufficient time period.
As with all wine "rules," you need to experiment to find which wines and which serving methods gives you the greatest enjoyment.