Teas can generally be divided into categories based on how they are processed. There are at least six different types of tea: white, yellow, green, oolong, black, and post-fermented teas of which the most commonly found on the market are white, green, oolong, and black. Some varieties, such as traditional oolong tea and Pu-erh tea, a post-fermented tea, can be used medicinally.
After picking, the leaves of Camellia sinensis soon begin to wilt and oxidize, unless they are immediately dried. The leaves turn progressively darker as their chlorophyll breaks down and tannins are released. This enzymatic oxidation process, known as fermentation in the tea industry, is caused by the plant's intracellular enzymes and causes the tea to darken. In tea processing, the darkening is stopped at a predetermined stage by heating, which deactivates the enzymes responsible. In the production of black teas, the halting of oxidization by heating is carried out simultaneously with drying.
Without careful moisture and temperature control during manufacture and packaging, the tea may become unfit for consumption, due to the growth of undesired molds and bacteria. At minimum it may alter the taste and make it undesirable.
Tea is traditionally classified based on the techniques with which it is produced and processed.
White tea: Wilted and unoxidized
Yellow tea: Unwilted and unoxidized, but allowed to yellow
Green tea: Unwilted and unoxidized
Oolong: Wilted, bruised, and partially oxidized
Black tea: Wilted, sometimes crushed, and fully oxidized
Post-fermented tea: Green tea that has been allowed to ferment,compost