There are hundreds of different types of olives in the world, of various sizes, shapes, and colors. There is some confusion about "green" vs "black" and how that correlates with "ripe" vs "unripe". In researching this, I found that the most reliable sources were from scientifically-reviewed university sources, such as this publication and this one from the University of California. I recommend them if you are interested in more detailed information.
Most olives start out green, and darken to black (or almost black) as they mature, passing through rose-colored and brown on the way. Strangely, this does not mean that green olives are never considered to be ripe, or that black olives are always ripe!
Green olives are considered to be "green ripe" when they have reached full size, but have not yet begun to change color. When squeezed on the tree, they produce a creamy white juice, and the flesh is very firm. When the olives are "turning color" (between green and black) the flesh remains firm, but gradually softens as it becomes darker. "Naturally black ripe olives" have become fully ripened on the tree before picking, and release a reddish-black juice when squeezed. They are easily bruised. Since the riper the olive, the more oil it has, these olives are usually used for olive oil.
So, green olives can be considered ripe (green ripe), but what about the black ones? It turns out that "naturally black ripe olives" that have matured on the tree are too fragile for the canning process, which involves adding pressure that would turn the olives to mush. Canned black olives are usually green olives cured with lye and exposed to oxygen. The black color in these olives comes from oxidation.
Olives need to be cured to make them palatable. The curing removes some or all of a substance called oleuropein, which is very bitter. There are several ways to cure olives, including the use of lye, brine, salt, or water. Some are also naturally fermented. Each process takes a different amount of time and has a different effect on the flavor. Many of the traditional olives of the world use characteristic curing processes which contribute to their flavors.
Olives are mostly grown in regions with a "Mediterranean climate", with long, dry summers and cool, wet winters. Although most of the world's olives are grown in areas around the Mediterranean sea, there is a growing production of olives and olive oil in California, Australia, Chile, South Africa, and Argentina, all of which have area with this type of climate.
Health Benefits and Nutrition Information of Olives
Types of Olives from Peggy Trowbridge, About.com's Guide to Home Cooking