A mob (short for "Mobile objects" or "Mobile Object Blocks", or "Monster Or Beast" as a bacronym) is a generic term for any non-player entity whose primary purpose is to be killed for experience, quest objective, or loot. They tend to be aggressive (aggro: Alliance Horde) to all players.
The gaming term "mob" is in widespread use in the MMORPG genre. The term is not to be confused with its usual meaning in the English language, i.e. a large disorderly crowd or an organized gang of criminals.
Mobs are also known as bad guys, beasts, enemies, monsters, non-player attackers, and opponents. The World of Warcraft-specific term for a mob is creep which was coined by Blizzard and first used in Warcraft III.
In World of Warcraft, mobs differ from NPCs in that they are usually monsters that share a common name with several others rather than having a unique name or title (but, see boss mobs), are non-friendly to everyone, are non-interactive, and/or exist solely for the purpose of being killed. These are part of the main aspect of PvE, and are found throughout the game world. Note, though, that hostile faction NPCs are essentially mobs. There are also named mobs that are either quest related which are usually stronger than the surrounding unnamed mobs or rare mobs that are often elite.
In World of Warcraft, mobs differ from critters in that critters are primarily for ambiance, provide no direct experience and little or no loot, and are trivial to kill. There are subtle mechanical differences, such as critters cannot be targeted by use of the tab key (mobs can be).
The term was originally used in MUDs (Multi-User Dungeons/Dimensions/Domains), the text-based precursors to MMORPGs. In many MUDs, in-game objects are defined either as stationary item "objects" (ranging from fountains to wearable equipment that you could pick up) or "mobiles," shortened to "mobs" (this included any NPC, be they aggressive monster or friendly shopkeeper, even if they were not actually moving around due to STAY_ROOM or similar flags in their properties).