Anti-oxidants: Present in white, green and to a lesser extent in black tea: have various stated benefits.
Caffeine: Tea contains caffeine in widely varying amounts. The potency usually depends on the maturity of the leaves used, the region where they were grown, and how they were processed, shipped, and stored.
Tannin: The substance that gives tea its bitter taste and most of its color - 'stewed' tea is where much of the tannin has dissolved into the liquid. See Phenolic content in tea for the effect of tannin and related compounds in tea.
Theine: This compound is closely related to caffeine and is biologically the same.
Theophylline: Another compound in tea that is similar to caffeine is present in very minimal amounts; cocoa beans contain larger quantities.
Herbal teas: Herbal teas may not contain caffeine, theine or tannin. They will usually have various ingredients stated to have benefits or be contra-indicated with various health conditions.