As any tea drinker is aware, most types of tea, from green to black to herbal, contain various amounts of a group of chemical compounds collectively referred to as "tannins." But what does this mean?
Tannins are compounds found in virtually every plant on Earth (source). They play a role in the ripening of fruit: if you have ever bitten into an unripe apple or banana and experienced a bitter taste, it's tannins you are tasting (source). Likewise, if you have ever choked on an accidental swallow of unsweetened black tea, the tannins are to blame.
While for most people, the ingestion of tannins is harmless, some sensitive individuals may experience bowel irritation, kidney irritation, or liver damage upon consumption of extremely large amounts of tannins (source).
Additionally, even in moderate amounts, tannins do possess one negative side effect: they inhibit the absorption of iron found in vegetarian foods (source). This means that anemic individuals and vegetarians may want to stick to drinking herbal and Rooibos teas with meals, while enjoying black, green, Oolong and Pu-erh teas a few hours before and after meals.¬†
For the vast majority of mankind, however, tannins stand to do quite a bit of good: in scientific studies, they have been found to possess antiviral properties (source). And, much more importantly, as any tea lover knows, they add a wonderful dimension to the flavor of tea.