Of all the spices, probably the most ubiquitous is pepper. (Some might say it's salt, but I don't consider it a spice because it's a mineral, rather than a plant.) Green peppercorns, white peppercorns and black peppercorns come from the same plant - the Piper nigrum vine. The stage at which they are picked and how they are processed determines the colour.
The green peppercorn is the unripe fruit and it can be used fresh, dried or pickled. It is aromatic and hot, but not as strong as black or white peppercorns. Black peppercorns are also unripe, but the fruit has been processed, fermented and dried in a way that develops the flavour - making it more pungent than the green variety - and turns the exterior black and wrinkled. White peppercorns are made from the fully ripe fruit, which is processed and the exterior removed, leaving only the white core. White pepper is the strongest and hottest of the three, and a little goes a long way. It's used in pale sauces, where the dark specks from ground black pepper would be unsightly. The Piper nigrum vine can also yield red peppercorns (which should not to be mistaken for pink peppercorns - those come from a different plant), but these are very rare.
The pepper plant grows best in tropical climates, so can be found in places such as Vietnam, Thailand, Malaysia and other Southeast Asian countries, as well as parts of China.
Peppercorns - whether green, black or white - start to lose pungency once ground. For the best, truest flavour, grind the pepper just before using it.
Fresh green peppercorns can be difficult to find outside Asia, because they're perishable. In Hong Kong, you can find them in shops specialising in Southeast Asian products.
I like them best in a crab dish. Kill the crab and clean it, then chop off the large claws and lightly crack the shells. Cut the body into four or six pieces. Heat oil in a wok and deep fry the crab to partially cook it, then drain the pieces on paper towels. Pour off most of the oil from the wok, then place it over a high flame. Stir-fry sliced onion and two or three garlic cloves, then add the crab back into the wok along with a few clusters of green peppercorns. Season with salt and ground black pepper, then stir in some water mixed with cornstarch and cook until the sauce lightly coats the crab. Transfer the ingredients to a large clay pot, cover with the lid and simmer for about 10 minutes, or until the crab is fully cooked. Open the lid, add a chunk of butter and stir until it's completely melted, then serve.