Dark tea (also known as pu-erh) is the only fermented tea among all six types of tea. It owes its name to the deep brown color of its infusion. The leaves of pu-erh may or may not be oxidized, which depends on a manufacturing style, but all pu-erh teas undergo fermentation, which gives them their characteristic flavor.
There are two types of pu-erh: (1) raw pu-erh (also called green), which contains unoxidized leaves and undergoes natural post-fermentation; and (2) cooked pu-erh (also called black), which consists of oxidized leaves and undergoes accelerated fermentation.