http://en.wikipedia.org/wiki/Peppercorns
Green pepper, like black, is made from the unripe berries. Dried green peppercorns are treated in a manner that retains the green colour, such as treatment with sulphur dioxide or freeze-drying. Pickled peppercorns, also green, are unripe berries preserved in brine or vinegar. Fresh, unpreserved green pepper berries, largely unknown in the West, are used in some Asian cuisines, particularly Thai cuisine. Their flavour has been described as piquant and fresh, with a bright aroma. They decay quickly if not dried or preserved.