There are many types of olives, including variations within black and green olives. Kalamata olives are neither; the large, smooth olives are actually a deep purple. They are used in different recipes, and green ones are almost always the ones used in martinis.
Green olives are picked and cured when they are unripe. They are usually brine cured, soaked in a solution of salt and water, to make them less bitter.
Kalamata olives are generally rather salty. They are also brine cured. Some black olives are dry cured, covered with lots of salt. You can tell dry-cured olives by their wrinkly skin.
Green Picholine olives
Picholine olives are the small green olives often used in French cooking. The true Picholine olives, from Provence, are brine cured, then marinated in herbes de Provence and coriander.
Pitted Green Olives
Large Spanish olives, also know as Manzanilla olives, are the ones that are pitted and stuffed with things like pimiento or garlic. Look for Spanish olives stuffed with lemon or orange for a twist on a traditional martini olive.
Cooking With Olives
Kalamata olives are good for cooking, or making into tapenade, because they are easy to pit. Their large size makes them easy to handle, and the pit pops out.