Green olives are the unripened fruit of the olive tree. (All olives ripen from green to black, through intermediate stages of reddish, brown, and purple.) Green and black olives aren't necessarily different varieties of olive, as olives are harvested and cured for the table at all stages of ripeness. Examples of green olives incliude Picholine and Lucques. They tend to be more mild than black olives, but can be difficult to pit.
Black olives usually make a fine substitute.
At the market, olives should be unbruised, clean, and firm (if brine-cured). Brine-cured olives should also be plump, with smooth, shiny skins and moist interiors.
To pit stubborn olives, put the olive on a work surface, set the flat side of a heavy chef's knife on top, and give it a good whack. The force splits open the olive and frees the pit. Be sure to wipe the knife blade frequently because it gets oily—and very slippery—after splitting a few olives.
Always keep olives moist, either in brine or sprinkled with olive oil and store in the refrigerator.