Szechuan Peppercorns are called peppercorns because of their flavour, not because of any actual relation to pepper. They are actually dried berries from a tree (a shrub, actually) which grows in parts of Asia called the "Prickly Mountain Ash" tree.
The shrub is deciduous, with brownish-black bark. It's called Prickly because the trunk, branches and leaf stems have thorns on them. It's related to the North American Prickly Ash tree, but the berries of this North American cousin don't have any taste and consequently aren't used in cooking. Their berries are dark, blackish-blue instead of red as the Asian varieties are. Prickly Mountain Ash is also related to citrus trees.
The Prickly Mountain Ash shrub produces small yellow flowers from April to May, and then pods that ripen to red by September, and then mature further to brown.
Inside the pods, the berries (about 1/4 an inch wide -- 4 to 5 mm) dry to a rust-colour, and split open to reveal 1 small (about 3 mm wide) brittle, shiny-black seed.
In Chinese use, the seeds are often just discarded before grinding the dried berries. Most of the flavour and aroma is in the berry itself, rather than in its seed. The seed, in fact, has a slightly bitter taste.
The dried berries are usually ground for use as a spicy powder. The powder ends up a light yellowish-green colour.
In Japanese use, the berries are ground in a mortar made from the wood of the tree to add to the flavour of the ground peppercorns. In Japan, the peppercorns are mostly sold already ground.
The spice has a sharp pepper taste with a bit of a tang to it. The taste starts off a bit bitter, then you taste the heat, then a citrusy aftertaste comes in. Some Westerners, though, don't like this aftertaste -- to them it seems to be soapy or like camphor. Some varieties of Szechuan Peppercorn have an anisey fragrance as well.
Though not as pungent as black pepper, and having only a very mild heat, Szechuan Peppercorn is prized both for its aftertaste and for the way it numbs your tongue, like an anaesthetic.
Prickly Mountain Ash trees are also grown in China, Japan, Korea and on the West Coast of India (see entry for Tirphal.) As the peppercorns are one of the few spices that can be grown in the Himalayas, they are also very important in cooking in Tibet and Bhutan. In Korea, there are two main varieties grown. One, which the Koreans called "chopi", is the same as that used by the Japanese and Chinese. The other variety which the Koreans call "Sancho" (whose scientific name is Zanthoxylum schinifolium), has seeds which aren't as bitter and which in fact have some flavour and aroma to them, and so are used.
Szechuan Peppercorns have, however, been banned in the US since 1968. They are also banned in Australia. Agricultural authorities in those countries suspect that the peppercorns can carry a canker caused by bacteria (Xanthomonas axonopodis pv. citri) that might be harmful to other members of the citrus family, though no actual case of this has been identified. Neither the bacteria nor the canker are harmful to humans. Though both the bacteria and the canker can be killed by heating the peppercorns to 140 F (60 C), that degree of heat destroys the taste of the peppercorns and renders them useless. The peppercorns are not banned in Canada or the UK, as there are no citrus trees there.
Szechuan Peppercorn can be bought whole or ground. When sold whole, it will sometimes will be packaged with bits of stem or thorn; remove and discard those.
The leaves from the trees are also used in Japanese cooking. See entries on Kinome and Sansho.
Whole Szechuan Peppercorns can be used in soups or stews, etc.
In Japanese cooking, grilled eel (unagi) usually has ground Szechuan Peppercorn sprinkled over it.
Chinese food fans say that mere black pepper is a lifeless substitute, but there you go. Lemon Pepper, if you feel like making it, might be a better substitute.
Store in sealed containers away from light.
Native to Szechwan province in China.
In Chinese, Szechuan Peppercorn is called variously jiao ("pepper"), shan jiao ("mountain pepper"), hua jiao ("flower pepper") or qin jiao ("Chinese Pepper").