Tea plants (Camellia sinensis) produce astringent tasting polyphenols called tannins, and tea brew generally contains somewhere around 0.01% tannin. There are thousands of different tannins and at least two thousand of them are know to have a flavon structure and thus called flavonoids. One type of these flavonoids, and the most prevalent in tea, is a group of flavonoids called catechins. Catechins have a bitter / astringent taste, and are believed to be the primary reason tea has been found to have many health benefits (such as reduction in ischemic heart disease, cancer resistance, lowering blood cholesterol, and help controlling allergy). Much of the catechin in tea leaves convert into theaflavin, and thearubugen if the leaves are oxidized (turned into black tea as opposed as kept as green tea). This conversion is responsible for the color change in the teas.
Don't be to quick to switch to green tea for the the health benefits though. Theaflavin and thearubugen are pretty much just strings of catechins and are being found to have the same activities as lone catechin molecules. Tea brew also contains about 0.002% caffeine, which also has a bitter taste.