Green olives are unripe olives, black olives are ripe olives. Unripe olives contain very high levels of oleuropein, which is especially bitter and thus green olives cannot readily be eaten. Ripe olives have little, if any, of this compound.
To make green olives edible, they have to be cured (typically using a lye-based process for speed, but can be done with plain water over a longer period of time) to remove the oleuropein, and thus making them edible. After curing, they are typically kept in a salty brine to extend their shelf life.
Black olives do not need the curing process, but can be brined in either a salty water brine, or oil, or no brine at all via pasteurization.