All tea types, except herbal, are brewed from the dried leaves of the Camellia sinensis bush. The level of oxidation of the leaves determines the type of tea.
Green tea is made from unoxidized leaves and is one of the least processed types of tea. For this reason, it contains the most antioxidants and beneficial polyphenols.
Black tea leaves go through a process of wilting, bruising, rolling, and oxidation. It has caffeine as well as other stimulants and antioxidants.