All tea comes from the plant Camellia Sinensis. The types of tea that can be found are white, green, oolong, and black. All the teas from the grocery store are derived from these types and flavor is added. The type of tea (white, green, oolong, or black) is determined by when the tea is harvested, or the time the tea is picked. The amount of oxidation that occurs also plays a role. Oxidation occurs when the picked tea leaves are exposed to the air. The leaves then react with the oxygen in the air and start to turn brown. This is similar to what happens when leaves fall from a tree. Another determining factor of the type of tea is the processing that it goes through.