The only difference between green olives and black olives is ripeness. Unripe olives are green, whereas fully ripe olives are black.
Olives are cured or pickled before consumption, using various methods including oil-cured, water-cured, brine-cured, dry-cured, and lye-cured.
Green olives must be soaked in a lye solution before brining, whereas ripe black olives can proceed straight to brining. The longer the olive is permitted to ferment in its own brine, the less bitter and more intricate its flavor will become. Green olives are usually pitted, and often stuffed with various fillings, including pimientos, almonds, anchovies, jalapenos, onions or capers.
References : http://homecooking.about.com/od/fruit/a/olivevarieties.htm