All tea comes from the same plant (camellia sinensis) and like most plants, its leaves are green. Green tea preserves that green color and freshness. China and Japan produce the highest quality green teas, though many other countries are now producing it as well. Green tea usually produces a light body with mild astringency and a vegetal/grassy character.
Black tea is tea leaves that have been quickly and heavily oxidized. This gives the leaf a dark appearance and significantly changes the aroma and flavor.
Oxidation is the natural chemical reaction carried out by enzymes within the leaf that begin once they are exposed to air (like an apple turning brown when cut open).
Though all teas (other than green tea) go through some degree of oxidation, black tea is the only one that is defined by its high level of oxidation.
