Green peppercorns are true peppercorns of the Piper nigrium flowering vine plant, which originated in India and has been used since prehistoric times. Green peppercorns are really unripe black peppercorns. These are often preserved in brine or vinegar and served in pickled form. In dried form, they don't last very long and have to be used quickly. They're commonly found in Thai and other Southeast Asian recipes and have a fresher flavor than their black counterparts.
Black peppercorns are the most commonly used peppercorns, and are ground up and put in shakers next to the salt. These are green peppercorns that have been cooked in hot water and then spread out in the sun to dry. The ancient Egyptians used black peppercorns in their mummification processes, and it was well-known by the ancient Greeks and Romans, who used it as a spice. In Southeast Asia, black peppercorns were traded as currency. It remains the most widely-traded spice in the world today.
White peppercorns are black peppercorns that have had the black skin removed, so technically, they are just the seed of the fruit. They are less pungent and more earthy in flavor than black peppercorns, and are commonly used in white sauces and mashed potatoes due to their color as they don't visibly stand out like black pepper.
(Image: Spice and Tea)