Tea (Camellia sinensis) leaves soon begin to wilt and oxidize after picking. The leaves turn gradually darker because chlorophyll breaks down and tannins are released. This process, enzymatic oxidation, is called fermentation in the tea industry although it is not a true fermentation as there is an external/added element to trigger off the oxidation. This is the process that creates the major difference in different kinds of tea, eg black tea being fully fermented, green tea unfermented, oolong partially fermented.