The spice we know as Sichuan peppercorn isn't actually part of the pepper family, although it is used in the food of Sichuan (as well as in a few other cuisines). The spice is very fragrant but the effect on the tongue is less a flavour than a tingling sensation that numbs. The Sichuan peppercorn is the fruit of a type of prickly ash that belongs to the citrus family - and because it can carry a citrus canker virus that has the potential to decimate the citrus industry in the United States, it can't be imported into that country unless it's been irradiated.
When buying Sichuan peppercorns, look for a bright, reddish colour; if they're dull and brown, they won't be very aromatic.
To enhance their fragrance, Sichuan peppercorns are usually toasted by being stirred in an unoiled pan; or else they can be lightly fried, with the spice imparting its numbing sensation to the oil used. The fragrance comes from the dried husk rather than the small, shiny seed within, which is often discarded.
In Sichuanese cooking, the peppercorns are usually combined with chillies to produce the distinctive ma la (tingling/numbing and spicy) flavours we associate with that province's cuisine.
Most people are familiar with the peppercorn in the popular mapo doufu. There are many versions of this dish - some places prepare it with pork, others with beef, and it can also be vegetarian, but the constants are bean curd (of course), Sichuan peppercorns and chilli. I like to toast Sichuan peppercorns then finely grind them, sieve out the seeds, mix the powder with salt and a bit of sugar, then sprinkle over roast chicken.