Decanters have been used for centuries and have changed very little from their original form. Currently, almost all decanters are glass, but in the past metal, decanters were also common for serving wine. Decanters are used both to remove sediment and to create an aesthetic dining experience. The use of a decanter differs depending on the type of wine you are drinking.
Decanting a young red wine allows it to aerate or breathe, which some people believe will soften the tannins and improve the flavor profile of the wine. For this reason, decanters are usually designed to allow a large amount of surface area so that as much of the wine as possible is exposed to the air.
Mature wines (generally reds) may contain sediment or small particles. Sediment forms when tannins and other compounds join into chains as the wine ages. Having the bitter tannic elements fall out of the wine makes it taste smoother and allows other facets of the flavor to shine through. However, you certainly don't want them falling into your glass! When opening an older red wine, allow it to stand upright for several hours (24 if possible) to allow all the fine particles to settle to the bottom. Then when you are ready to serve the wine, open it and pour slowly and carefully into a decanter. Watch for the sediment to appear in the neck of the bottle so that you can stop pouring before it escapes. The decanted wine should be free from sediment. Serve it immediately--older reds don't need aeration and will only fade if left out too long.
Have you ever accidentally broken a cork and ended up with a cork in your bottle? To avoid having that cork end up in your glass, decant the wine first. You can pour it through a strainer if there are small bits of cork, but if you've just dropped a single large bit of cork into the bottle you can probably manage to decant the wine without the cork in it.
In addition to being a very practical addition to any wine-lover's glass collection, decanters are also a beautiful way to serve wine. The color and clarity will be elegantly displayed in a quality decanter. Depending on your tastes, you can choose from stoppered designs, simple pitcher shapes, artfully elongated styles or more unusual designs. Many Riedel wine glass collections have a decanter designed to complement them. When choosing a decanter, feel free to pick the shape that looks best to you. No particular shape is better than another for aeration purposes.