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 (the most recent example is the second-to-last group prior to High Warlord Naj'entus). 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.
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 back yard. 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.