Trash mobs are usually easier to kill than bosses, although there are some famous trash pulls which are actually harder than the boss they precede (a more recent example would be the opening pulls of Bastion of Twilight which has two patrols that are inter-meshed with each other). Players usually don't like trash mobs since they are viewed as a tedious and time-consuming preamble to boss fights. Nonetheless, trash mobs in instances usually give reputation and have some useful drops, frequently some crafting material which cannot be obtained otherwise.
Trash mobs comprise the non-boss denizens of any dungeon or raid, and thus the vast majority of instance inhabitants. They provide additional loot, an opportunity for groups to warm up, and add flavour and tension to what would otherwise be almost entirely empty instances. Patrols are trash mobs that perform the additional function of keeping players on their toes, thanks to the chance of them attacking players unawares, or forcing them into a larger confrontation than they were prepared for.
Despite the dull title, trash mobs frequently feature specific mechanics, with melee and ranged types, dedicated healer mobs, and mobs with dangerous abilities or safeguards that require specific (albeit usually minor) strategies to defeat. The selection of trash mobs usually changes over the course of an instance, with grunt-level trash just inside the portal, and more impressive examples as players near the final boss. For example, the upper levels of an instance may feature mortal trash mobs, slowly changing to more demonic forms as the players progress, with demons taking over guard duties entirely at the deepest levels. Trash mobs sometimes rise almost to the level of mini-bosses, with a single mob occupying the group for a short while.
The term trash mob derives from the term yard trash which was coined in the late 1990s MMORPG Everquest. In that game, a low-level dungeon, Unrest, was composed of a house which contained several bosses, surrounded by a front, side, and backyard. In the yards, there were yard trash. Lower level players shouted for groups to kill yard trash for experience either before they were high-leveled enough to kill the bosses or when all the bosses were camped.
- During the 2007 Blizzcon Dungeon & Raid panel, Blizzard came up with the backronym "Tactically Random A.I. Spawn Heuristic" for TRASH.