The olive is a bitter, tart, and/or savory fruit. Many varieties are grown, and a large number of preparation methods exist. Most olives are simply pressed for oil.
The fruit is naturally quite bitter; processing is necessary to reduce this bitterness to tolerable levels, though a noticeable amount remains in gourmet olives. Fermentation and osmosis are methods of destroying and extracting this bitterness, respectively.
Osmosis methods resemble pickling or salt curing, but are carried further since the goal is the removal of bitter water-soluble chemicals, rather than water alone. Olives are bruised and then either leached repeatedly in brine or packed in salt for long periods of time.
The green olive and black olive are the same plant; green olives are pickled before ripening, where black olives are pickled after being ripened.