Antioxidants have to be one of the most over-hyped classes of chemicals. They are found in nearly all plants, and thus, in most plant-based foods, and of course, in tea. Wikipedia has a good page on antioxidants, which paints a comprehensive and fairly accurate picture of these compounds.
Tea companies are guilty of exaggerating the health benefits associated with the antioxidants in tea, and companies marketing nutritional supplements have taken this misinformation even farther.
Pictured here is (+)-Catechin, one of the simpler antioxidants found in tea.
For a briefer read than Wikipedia's article, more oriented towards the topic of tea, I recently published an article on the antioxidants in tea on RateTea, which describes and discusses the antioxidants in tea and their impacts on health. The picture painted by my surveying of the recent scientific research on this topic is very different from what you hear in all the marketing of tea companies. Yes, tea is antioxidant-rich, but these chemicals are not necessarily as good for you as you might think.