Tannins are polyphenols found in the grape's skins, seeds, and stems, as well as in oak barrels. They give a drying, astringent sensation in the mouth — similar to drinking strong black tea. Tannin is almost exclusively a property of red wines, since white wine ferments without skin contact.
Tannins are called ripe if they feel velvety soft, and unripe if they seem hard and bitter. A high tannin concentration is not a fault — it is a structural element. With time in the bottle, tannins soften and integrate with the other flavour components. Grapes with naturally thick skins, such as Cabernet Sauvignon, contain more tannin than thin-skinned varieties like Pinot Noir.