Tannins are polyphenols, the primary source in wine being the skins of the grapes. Stalks also contain tannins of a greener, harder nature and nowadays are, with some notable exceptions, generally excluded from the winemaking process. Oak is another source of tannin, and wines matured in new or young barrels, or otherwise oaked, will absorb tannin from the wood.
Tannin is mostly detected by tactile sensations, particularly making the teeth and gums feel dry, furry and gritty. The sensations can be mouth puckering, and after tasting wines high in tannin, you want to run your tongue across the teeth to clean them. Hard, unripe tannins also taste bitter and 'green'. Tannin is a key component of the structure of classic red wines and gives 'grip' and solidity.
Tannin binds and precipitates protein. This is one of the reasons why most red wines match red meats and cheeses successfully. This combination causes wines containing tannin to congeal into strings or chains as it combines with protein in the mouth, and thus our perception of tannin in wine will change if we keep it in the mouth too long. To observe this, take a good mouthful of red wine low in tannin such as a Beaujolais, chew it for 20 seconds or so and then spit out into a white bowl. Now, repeat the exercise with a red wine high in tannin such as a Barolo. It will be observed that the greater the tannin level in the wine, the more the wine will have formed these strings.
Novice tasters often confuse the sensations of acidity and tannin. A classic traditionally made Barolo, which is high in both, is a good example to taste to distinguish between them. The tannin gives the dry, astringent sensations on the teeth, gums, and even hard palate. The acidity produces the tingling sensations on the sides of the tongue and cheeks.
With maturation and ageing, tannins will be precipitated, and will form a sediment in the bottle, or glass if the wine has been carelessly handled at service.