The practice of drinking tea was invented in Yunnan; of all Yunnan teas, pu-erh is the most ancient. It is the original form of dried tea, preceded among all varieties only by the brewing of fresh leaf off the bush. The process of drying the tea is involute compared even to most oolong; it bears the hallmarks of a method devised by long practice and incremental improvement, as opposed as conceivable to a modern, efficient industrial process — and indeed, the better teas are still processed entirely by hand. It is an invaluable process, however; it is almost certainly this that gives the tea its famous ability to age well. A typical black tea, bought in the shop, will lose its flavor within three months of purchase; it degrades progressively from the moment it is dried. Pu-erh can be stored indefinitely, and what's more, raw pu-erh (very slowly) ferments during storage and deepens in complexity of flavor with no known upper limit. (The inferior drying method is also what gives black tea that abominable bitterness which prevents its reuse.)
The best pu-erh — according to some, the only real pu-erh — comes from wild trees, preferably very old ones, uncultivated by human hand, picked in the wild; this again is unlike most tea, which is grown on heavily cropped bushes. (The fact alone that the normal term is »tea bush« instead of tree should indicate about how heavily.) There is, unfortunately, a great vogue for bald-faced lying on this point, and it is difficult to be certain that the tea marked »wild arbor« or »ancient arbor« is in fact what it claims to be. Two characteristics that will be found in a true wild tea are a scent of camphor, and serrated edges on the leaves; however, neither of these is a conclusive proof either. Caveat emptor, in other words.
There are two types of pu-erh, quite different:
Sheng, »raw« pu-erh, is that described by my esteemed colleague above; it is improved by storage; its cost when long kept would make a vintner weep and swear he had chosen the wrong profession. It needs no further comment than those so ably provided above, except perhaps to note that since that was written, there has been a huge boom of interest in pu-erh in mainland China, no doubt tied to the improved economy of many Chinese, paired with nationalistic sentiment. (The Chinese are allegedly now the largest buyers of vintage French wine; pu-erh is the nearest domestic equivalent.) Previously, the main connoisseur market for pu-erh was Hong Kong, followed by Taiwan, and many of the big tea merchants are still based in Hong Kong.
Shu, »cooked« or »ripe« pu-erh, is made from the same leaves, albeit usually those of lesser quality. This was invented in an effort to hasten the aging process and emulate the flavor of a long-stored raw pu-erh (hence the two names of the types). It did not really succeed in that regard, I would say, but it has a great deal of charm of its own. (It is my own favorite type of tea.) Traditionally, aging was understood to do nothing for it, and it was also exactly contrary to the point in the first place. Ripe pu-erh is normally much cheaper than raw, on account of the low grade of leaf and lack of aging.
Ripe pu-erh brews a dark liquor, ranging from a ruddy brown with an opacity almost like that of coffee to a limpid deep amber. It will typically steep extremely fast, and brew 3-5 pots before being washed out. The liquor becomes progressively clearer and lighter in tint, and there is a marked change in flavor with each pot. The first pot, despite perhaps having steeped for 30 seconds or less, will have a very strong taste; this is usually described as »earthy«, and it is somewhat sour especially on the aftertaste.
In recent years, the last decade or so, there has been an increase in interest in aging ripe tea also, driven no doubt by the aforementioned purchasing boom, but I for my part am inclined to agree with the wisdom of the past: it appears to have no effect, on the taste or otherwise.
There are also many shapes in which pu-erh is pressed. The four most common forms are:
Cake: Or »bingcha«; a round cake with a depression in the bottom, it is formed by wetting dried tea leaves and placing them, typically along with a ticket showing the factory and perhaps provenance, in a cloth which is then twisted shut (hence the pit in the cake; this is where the twist of cloth sits) and pressed in a form, traditionally stone, now often a steel machine stamp. The cake is then wrapped in a paper wrapper bearing various marks of production. The cakes traditionally weigh 357 grams, which I understand is equivalent to ten taels (but I know nothing of Chinese weights, frankly); they are now often produced in 400g or 500g weights for more sensemaking in the metric system. The cakes in turn are packed in tongs of seven cakes, wrapped in bamboo leaves; these are a common unit of sale to the serious tea collector. The best pu-erh tea is pressed into cakes.
Brick: What it sounds like, a rectangular brick. Brick material is usually of lower quality than cake material, and it was traditionally sold to places like Tibet, with the understanding that those blasphemous bastards were going to put yak butter in it anyway so who cares if their tea is any good? Much brick tea is still sold to places like Tibet. And they still put yak butter in it. Jesus Christ. China was right to invade them. Anyway, before all that the brick was also the earliest pressing shape, and the older ones have Chinese characters embossed into them, or even elaborate pictures. You still sometimes come across ones pressed with characters.
Tuocha: »bird's nest«, »nugget«, or »knob« shape tea, the shape of these is very characteristic, resembling somewhat a fat mushroom cap; like the cake, the tuo has a depression at the base. They are usually made in the weights 250g, a more traditional size, and 100g, which so far as I know has always been the standard for the export market. There are also 50g tuos; concerning smaller sizes than that, see below. They are normally not nowadays of as good quality as cakes, but better perhaps than brick. (Note however that these are all generalities; there are assuredly amazing tuos, and you can obtain a shitty cake with consummate ease.) There are many variant shapes of what are essentially tuocha, like the »mushroom«, which has a sort of stem, and the scored »melon« shape, which originated as the shape for that tea which was sent to the Chinese emperor as tribute. (This tea of course was of a very high grade, and the shape retains some prestige for that reason.)
Mini tuocha: Like tuocha but small (3-5g), these are basically pre-apportioned so you don't have to spend time prying the tea off. Mini tuos are usually only made with ripe tea, and practically always from the lowest-quality leaf. They are still a good, easy, and inexpensive choice (besides which you can find better-quality ones). One is appropriate for a small pot (such as a gongfu pot); two are good for a liter pot. Estimate one per half-liter of tea.
It should be noted, finally, that when I say »low-grade leaf« or »low quality«, this is a relative measure, and it is basically impossible to find a pu-erh containing leaf of a quality as low as that of ordinary bag tea, which is the bidi of the tea world. You need not, therefore, be afraid of getting an undrinkable tea; if you can drink Yellow Label, you can drink anything.