Tannin – What is It?

The term tannin is talked about a lot in wine circles and if you are wondering what it means, think of a strong cup of tea. Another example is if you have ever accidentally bit into a grape seed. That woody taste is tannin.

Sources

Tannins are a family of natural organic compounds that are found in grape skins, seeds, and stems. Additionally, during the aging process oak barrels infuse tannin into the juice. They are an excellent antioxidant and natural preservative; also helping give the wine structure and texture. Tannins provide an important flavor dimension in wine.
Winemakers have a good degree of control, and use that to enhance the final product. They use specific juice extraction techniques to reduce or increase the amount excreted. Specifically, they can very gently squeeze the grapes to extract the juice, whereby not releasing much of the tannin. The opposite is true as well. In the case of red wine, grape skin contact is longer, the crushing of grapes is more violent, and barrel aging is longer...resulting in a stronger tannin dimension in the wine.
In concentrated quantities, it will cause occasional pucker sensation in the mouth and back of the throat. This is sometimes accompanied by a bitter aftertaste, which is referred to as tannic. Visually, tannin forms part of the natural sediment found in the bottom of the bottle.
Red wines with little tannin should be drunk young. However, a red wine that should age and improve for perhaps three or more years requires a lot of tannin. As the wine ages, the tannin softens and becomes less noticeable.

from: http://www.winedefinitions.com