Fear is a core warlock ability learned at level 14. Fear causes a single enemy to flee in fear, effectively disabling it temporarily. This can be a powerful, though dangerous, tool that is integral to warlock crowd control.
Fear has a damage limit. If the damage limit is reached, the Fear spell will break, regardless of the length of the spell. As of Cataclysm Glyph of Fear makes your Fear cause the target to tremble in place instead of fleeing in fear.
Due to the diminishing returns rule, repeated castings of Fear increase the victim's ability to resist it for players, but not for mobs. A mob can be repeatedly feared indefinitely.