Tannin is a substance found naturally in many different plants, most notably in grapes, tea leaves, and oak. The word comes from the historical practice of extracting this compound from oak bark to tan leather, although in the modern world, synthetic forms or other chemicals are usually used instead.
This substance is found in the skins, seeds, and stems of grapes. Wine that is fermented while still in contact with these parts of the grape — red wine — absorbs some of the tannin, which lends a distinct character to the wine. Because the tannin in the seeds of grapes is particularly harsh, wine is usually crushed rather than pressed, to minimize its absorption. Many wines also absorb some from the oak barrels they are aged in, which lends additional flavor to the wine.
Tannin is crucially important to the complexity of a wine and how well it will age over time. Wines that have high amounts tend to age best for long periods of time, such as those made from the Cabernet Sauvignon, Nebbiolo, and Syrah grapes. When young, these wines often seem searing, creating a sensation of dryness in the mouth, particularly the roof of the mouth. The most tannic of wines cause the mouth to pucker, which is not necessarily a pleasant experience.
Over time, however, the tannin acts as a preservative and mellows, helping the wine to grow into its complexity and become truly exceptional. A well-made, high-tannin wine, which may be virtually undrinkable when young, can become a masterpiece of a wine later in life. That said, some wines are simply too tannic, and by the time their astringency fades, the wine will have passed its prime.
One thing that can help wines that possess a high amount of tannin more palatable is to combine them with high-fat and protein foods. Milk fits this role ideally, helping to soften the sharp edges of the drink. This is one reason so many people add milk to strong teas, which are also very high in tannin. Good cheeses or a hearty cream sauce can turn a wine that at first blush appears too tannic into the perfect pairing.
In general, the trend in winemaking is towards wines with less tannin than 50 years ago. These wines are easier to drink upon purchase or within a short year or two after buying them, and so are more suited to the new global wine market. To be well-rounded in the world of wine, however, it is a good idea to sample a number of wines with higher levels to taste the nuance that this substance can add.