Pu-erh tea (普洱茶) or pu'er tea, is a variety of fermented tea produced in Yunnan province, China. Fermentation is a tea production style in which the tea leaves undergo microbial fermentation and oxidation after they are dried and rolled. This process is a Chinese specialty and produces tea known as Hei Cha (黑茶). The most famous variety of this category of tea is Pu-erh from Yunnan Province, named after the trading post for Hei Cha during imperial China. Good Pu-Erh tea is made of tea leaves harvested of ancient tea trees (Gu Shu). Initially it was assumed that thoes trees are just very tall Assam bushes but recent DNA test showed that it's an entire new species. The discoverer named this plant Camellia taliensis to honor the Dai (Thai) minority which uses this plant traditionally to produce tea.
Pu-erh traditionally begins as a raw product known as Mao Cha (crude tea) and can be sold in this form or pressed into a number of shapes and sold as Sheng Cha (raw tea). Both of these forms then undergo the complex process of gradual fermentation and maturation with time. The recently developed Wo Dui process pioneered by both the Menghai Tea Factory and Kunming Tea Factory has created a new type of pu-erh tea that some traditionalists dispute the legitimacy of. This process involves an accelerated fermentation into Shou Cha (ripe tea) which is then sold loose or pressed in various shapes. All types or pu-erh can be stored for maturity before consumption and that is why it has become common for the products to be labelled with year and region of production.