What is Pu-erh tea?

Pu-erh tea belongs to a category of Chinese tea called Compressed Tea. Whole leaves typically compressed into various shapes: cakes, bricks etc. This assists this tea to continue fermenting after production has completed. In this respect Pu-erh tea "ages", like for example wine or cheese. 

Many people invest in Pu-erh tea, like they might invest in wine. They purchase quantities of tea and let it "mature" and increase in value with time.

Note that this characteristic is at odds with requirements for other tea varieties, where a premium is paid for "freshnes", related to lack of age!

There are also two major varieties of Pu-erh teas:

1. Pre-fermented Pu-erh tea: it is pre-fermented and ready to be used immediately after production
2. Green (raw, Shen) Pu-erh tea: the tea leaves are compressed into cakes and the fermenting process starts after the production, naturally fermenting within the tea cakes. The teas are not normally used until two years after production. The newly produced Pu-erh (under-fermented) is too rough and strong for most of people to accept.

Fermentation: further background:

In the context of tea production fermentation is similar to the process that occurs naturally when fruits discolour (ie an apple turns brown).  This is known an enzymatic oxidation , where oxygen reacts with an enzyme in the cells of the  fruit ( in this case tea leaves). The same process is used as the basis for fruit preservation (ie preserved peaches etc).

In tea production, the enzymatic  oxidation process  is halted at the desired stage by suitable application of heat before the leaves are fully dried ( ie through mild steaming or frying of the leaves).

Pu-erh tea is processed such that  fermentation is allowed to continue after processing has been completed - aided by the compacting of the tea leaves.