Black teatea made from leaves more heavily oxidized than the white, green, and oolong varieties. It is generally more flavorful and caffeinated, though perhaps not as beneficial to health as the other brews.
While green tea usually loses its flavor within a year, black tea preserves its flavor for several years (over 50 years in the special case of Pu-erh tea). It has thus long been an article of trade, and compressed bricks of black tea even served as a form of de facto currency in Mongolia, Tibet and Siberia in the 19th century. Black tea was traditionally the only tea known to western culture; though green tea has been catching on somewhat in the U.S., black still accounts for over ninety percent of all tea sold in the west.