Black pepper comes from the berries of the pepper plant. Black pepper, green pepper and white peppercorns are actually the same fruit (Piper nigrum); the difference in their color is a reflection of varying stages of development and processing methods.
Black peppercorns are made by picking the pepper berries when they are half ripe and just about to turn red. They are then left to dry which causes them to shrivel and become dark in color. Alternatively, green peppercorns are picked while still unripe and green in color, while white peppercorns are picked when very ripe and subsequently soaked in brine to remove their dark outer shell leaving just the white pepper seed.