A glass decanter is most often used for serving wine, though it can be used for other purposes as well. When used for wine, the glass decanter allows the wine's sediment to settle at the bottom, thereby preventing that sediment from ending up in the drinker's glass. It also allows the wine to "breathe," which means oxygen is allowed to briefly affect the flavor of the wine itself. Other materials are available for decanters, but a glass version will be less expensive than other materials and just as attractive or decorative.
Older wines tend to have sediment mixed in with the liquid, which can have an effect on the taste and texture of the wine when a person drinks it. Pouring the wine out of the bottle and into a glass decanter allows that sediment to settle, positively affecting the flavor of the wine and preventing the sediment from settling in the mouth. It is often recommended that older wines be decanted for this very reason, though young wines also benefit from decanting. When the wine is poured into the decanter, it is allowed to mix with oxygen, which can help mature the flavors of the wine very quickly. This enhances the drinking experience and prevents the wine from tasting too bitter or otherwise unpleasant.
Many wine enthusiasts eschew the glass decanter for a more expensive and beautiful crystal decanter, but for wine enthusiasts on a budget, a glass decanter works just as well. Crystal decanters tend to be highly decorative, with cuts and angles that make the decanter a visually appealing piece for display. Many glass decanters, however, are made more simply, with no designs or cuts to obscure the view of the wine itself. Crystal decanters can also be less decorative, and many enthusiasts recommend plainer designs to prevent the view of the wine from being obscured. This allows the wine drinkers to observe the tones and colors of the wine rather than the decorative filigrees of the decanter.
The process of using a glass decanter will vary according to the type of wine being poured into it. The process of separating the wine from the sediment can be tricky, and the easiest way to do this is to allow the wine to stand up vertically in the bottle for several days before pouring it. Younger wines should be decanted for a much longer period of time than older wines, as the flavors can be affected more dramatically as the new wine sits.