| Tannins are phenolic compounds that are produced by plants. The word tannin is an adapatation of "tanning", the process of converting animal hides into leather by steeping them in a solution made from the bark of plants such as the oak. In fact, tannins may be found throughout the plant kingdom, largely in woody plants.
Tannins are generally located on or near the exterior surfaces of plants, such as bark, leaf, stem, bud, and root tissues, though they may also occur in the seeds. It is thought that tannins function as a mode of defense for plants against herbivores--thanks to their chemical structure, tannins present a bitter or astringent taste when consumed due to their interaction with salivary glycoproteins. The tannins in wine--which contribute to both taste and color--are technically known as proanthocyanidins or PAs. They are soluble in water (or wine) when small, but because of their binding properties, often increase in size over time.
It is this property that allows red wines to age: as the years pass, the smaller, bitter tannins link up with each other (and with other compounds in the wine), grow larger, and eventually drop out of the wine as sediment. Thus, a wine with a lot of solute tannin may be bitter and dark red in its youth; but after a few years, this bitterness may fade and the color may soften in successive shades from purple to brick. However, an appropriate amount of tannin may be integral to the taste of a wine: this tannic edge may "fade" so far as to throw the wine's overall taste out of balance. Therefore, while tannins may help mask a wine's acidity in youth, an older wine with fewer solute tannins may seem harshly acidic.
Tannins find their way into wine from several sources. After the grapes have been pressed, a winemaker may allow the juice to have contact with the skin, stems, and seeds during a process called maceration, during which tannins are introduced into the wine, or "must" as it is called at this point. Once this process is complete (to the producer's satisfaction--white wines may only see a few hours of maceration, while reds can get weeks' worth), some winemakers will age their product in oak barrels. During barrel aging, additional tannins are leached from the wood and can help impart an "oaky" character to the wine. Winemakers may use new or old barrels, oak from France or America or elsewhere (I see more and more Slovenian oak being used), and barrels that are more or less charred. Each choice impacts the taste of the finished wine in some way.
Moving from the more to the less scientific, I would say that I enjoy the presence of tannin in wine, as it tends to lend "backbone" to a wine, giving it real character. Strongly tannic wines are best offset by dishes with high protein or fat content, such as a big steak or some quality cheese. Yum.
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In preparing this writeup, I made great use of the tannin webpage of Cornell Univeristy's Department of Animal Science, which was authored by Antonello Cannas. A native of Sardinia, he notes that "I have been educated to love tannins, especially when diluted with a 14% aqueous solution of alcohol and matched with aged Sardinian sheep cheese and some bread." A man after my own heart. | 
