Are you sure you want to know? Tannins are substances present in the seeds and stems of grapes, the bark of some trees, and yes, tea leaves. They are described as interfering with digestive processes, and until more effective synthetics were found, were used to tan animal hides and turn them into leather. That doesn’t sound very appetizing, does it?
Here is what Harold McGee, author of On Food & Cooking (Canada, UK) has to say about tannins:
"In the plant, tannins are defensive compounds that counteract bacteria and fungi by interfering with their surface proteins. They also deter herbivores by virtue of their astringent effect on the mouth and their interference with digestion. The sensation of astringency is caused by the ‘tanning’ of the proteins in the saliva and mucous membranes of the mouth; lubrication is reduced and the surface tissues actually contract."
Mr. McGee describes astringency, caused by tannins, as "that dry, puckery, constricting sensation that follows on a sip of strong tea or an assertive red wine, or a bite into less than ripe fruit."
Indeed, tannin is essential to the development of flavor in red wines. And the tannins produced in unripe fruits to protect them from predators disappear when the fruit ripens. But they do not dissipate in the production of most teas. In fact, they are indispensable to the production of both the flavor and color. In the process of constricting your tongue and (with apologies) the mucous membranes of your mouth, they create the impression of a full-bodied liquid. Tannins are also a pigment, responsible in large part for the tea’s color.
After tea leaves are picked, they are withered in fresh air or in a dryer, then mechanically rolled to crush the cells and mix various chemical components. Then they are fermented for one to three hours at about 80°F (27°C), during which time colorless and flavorless substances in the leaves are transformed into colorful and astringent tannins. Without the development of the tannins, the tea would lack color and its characteristic full-bodied flavor. Different varieties of tea are treated in different ways. Green tea has a pale yellow color and less complex taste because of a relative absence of tannins. Black tea is at the other extreme, with a much darker color and fuller taste. Oolong tea falls somewhere in between.
If you add milk to your tea, the tannins attack the proteins in the milk rather than those in your mouth, and the taste is much less astringent.
In moderation, tannins are not concentrated enough in tea to interfere with digestion. We’d say after 5,000 to 8,000 years or so of tea drinking (depending on whose legend you believe) and with humanity having continued thus far, you’d have to assume that tannins can’t be all that bad.