|Decanting simply means transferring the contents of a wine bottle into another receptacle, the decanter, before serving. It may sound silly but it works.
When you decant a bottle of wine, two things happen. First, slow and careful decanting allows wine, particularly older wine, to separate from its sediment, which, if left mixed in with the wine, will impart a very noticeable bitter, astringent flavor. Not to mention a noticable and inpleasant grit. Second, wine spends most of its life without oxygen so when you pour wine into a decanter, the resulting agitation causes the wine to mix with oxygen, enabling it to develop and come to life at an accelerated pace. This is particularly important for younger wine.
Decanting a young wine or one with no sediment is easy: Just pour it into the decanter. Let it sit for twenty minutes or so before you serve it, and you'll likely notice a dramatic increase in subtlety and complexity. If you have the luxury of time, continue tasting the wine over a period of hours. It may keep evolving and improving. And don't let anybody tell you that you should only decant certain types of wine (Bordeaux) and not others (Burgundy). I recommend decanting everything -- even white wine, if you feel like it.
Decanting an older with sediment requires a bit more finesse. For starters, the wine has had plenty of time to age on its own, so it doesn't need any artificial boost. You may even ruin it by overexposing it to oxygen before serving. Thus, you should decant older wine immediately before serving, before it begins to degrade. If you have the luxury of time you can take of bottle of wine with sediment and stand it up for a day or two before you want to serve it. This will allow the sediment to settle at the bottom of the bottle and careful pouring will avoid any of the grit getting into the glass.
Most people don't have that kind of time as wine decisions are often made on the fly. If this this the case, open the bottle carefully, trying not to distribute the sediment more than necessary. Pour slowly into the decanter. A flashlight or candle under the neck of the bottle can show you when the sediment begins to creep up the neck. Stop at this point. The wine you have decanted is ready to go. If you want, you can filter the remaining wine with cheesecloth or coffee filter. But you will notice a difference in the taste. Still you can enjoy it and make it a learning experience and train your palate.
A clear, crystal decanter allows you to see the wine at its best; overly decorated or colored decanters obscure the wine. Moreover, just as with your stemware, be sure that your decanter is spotless and free from any musty cupboard or soapy aromas. Rinse it with distilled water to remove any residual chlorine odor. And never clean your decanter with detergent, because the shape of a decanter makes it very difficult to get the soapy residue out. Instead, use a mixture of crushed ice and coarse salt -- they'll remove any residual wine without leaving behind any aroma of their own.