What is tannin, and is it desirable in wine?

Tannin is a natural substance that comes from the skins, stems, and pips of the grapes, and even from the wooden barrels in which many are aged. It acts as a preservative; without it, certain wines wouldn't continue to improve in the bottle. In young wines, tannin can be very astringent and make the wine taste bitter. Generally red wines have a higher level of tannin than do whites, because red grapes are usually left to ferment with their skins.                                       
Can you taste tannin?
The sensation of tannin begins in the middle of the tongue. Tannin frequently exists in red wines and white wines aged in wood. When the wines are to young, tannin dries the palate to excess. If their is a lot of tannin in the wine, it can actually coat your whole mouth, blocking the fruit. Remember tannin is not a taste: it is a tactile sensation.