Decanting your wine serves two main purposes. The first is to deal with sediment. It used to be common for wines to carry a high degree of sediment in the bottle which is unpleasant when served at the table. This is less common in today's wine, but still occurs occasionally - especially in older vintages. The second reason for decanting is to aerate the wine. Wine can take on musty, tannic or acidic qualities while bottled. Exposing the wine to air, even for just 20 minutes can change the quality of the wine to the extent where the taster will notice a more mellow effect, often exposing more of the subtle notes of the wine. A new style of decanters has burst on to the oenophile scene over the last few years. The most widely known brand is the Venturi. These devices handle the aeration of the wine much more quickly than a traditional decanter by passing the wine through a funnel, often creating a cyclone effect where the wine quickly absorbs oxygen. I've tried the Venturi and, while I'm not 100% convinced as to its benefits, it certainly does change the taste of the wine to the point where it may change your mediocre bottle into something a bit more pleasant if you're in a bind with insufficient time to properly decant.