Sometimes know as post-fermented or aged tea, Pu'erh (Poo-urr) tea is produced in China.  After processing the leaves are steamed and pressed into cakes and aged.  During this ageing process the tea slowly ferments and tiny microbiotic bacteria develop which affect the flavour.  Some of the rarest and most sought after teas in the world are Pu'erh teas and unlike other teas they gain value as they age.  There are two types of Pu'erh tea,  Sheng (raw) which is aged naturally over many years and Shou (ripe) which undergoes an accelerated fermentation process under controlled conditions.