There are dozens of varieties of olives, and both their size and flavor vary. All fresh olives, almost never seen for sale, are quite bitter, and the final flavor of the fruit greatly depends on how ripe it is when picked and the processing it receives.
Underripe olives are always green, whereas ripe olives may be either green or black. Ripe olives are oilier than underripe ones. Olives that are tree ripened turn dark brown or black naturally, and while the majority of these olives are used for oil, the rest are brine- or salt-cured; they are usually packed in olive oil or a brine or vinegar solution.
Dry-cured olives have been packed in salt, which removes most of their moisture and creates intensely flavored, dry, wrinkled fruit. These olives are sometimes rubbed with olive oil or packed with herbs.
Spanish olives are picked young, soaked in lye, and then fermented in brine for 6 to 12 months. When bottled, they're packed in a weak brine and sold in a variety of forms, including pitted, unpitted, or stuffed with foods such as pimientos, almonds, onions, and jalapenos.
Greek olives (the most popular being the dark-skinned Kalamata variety) and Italian olives are not picked until they are fully tree-ripened, and so have a dark purple color and are soft and juicy. They are then put down with rock salt in containers and left to stand for several months before being packed in olive oil and marketed. Kalamata olives are also marinated in a wine-vinegar solution.