Critters are typically harmless and low level. They are intended for ambiance, and predatory animals (felines, canines, etc.) are scripted to attack them when critters get too close, which gives a more realistic feel to the game.
Warlocks can summon some demons as minions, as well as enslave them for a time, but only the lesser ones.
Behavior and loot may resemble either humanoids or beasts. Humanoid demons like succubi and mo'arg assist each other when shot at and carry money and cloth. Bestial demons like felhounds and helboars drop body parts (junk) and can usually be skinned.
Depending on the respective element of the creature, a properly skilled miner (rock types) or herbalist (plant types/treants) may be able to extract junk items and sometimes elemental items from the corpse of the defeated creature. Works just like skinning. Provides no skill points, even when the text in the tooltip (used to determine if the corpse is exploitable) is orange.
More intelligent behavior than most other types of creatures. Pulling a humanoid, including with a ranged attack, will draw the ire of any nearby humanoids as well. When a group of humanoids behaves like this, they are referred to as being "social". Some demons, undead and even a few beasts are prone to this behavior also.
Nearly all humanoids tend to run away when they get low on health, usually attempting to pull other mobs in the process.
Moving devices created through a mix of expert engineering and magic.
Engineers can dismantle defeated higher level mechanicals and attempt to salvage parts (this works similar to skinning). These parts will usually be junk, but sometimes they are usable. A very lucky engineer may even find [Schematic: Jeeves].
The term undead refers to all creatures or entities that function normally despite having terminated life functions. Being bound by servitude to a greater entity appears to be a condition as well, since the Forsaken are instead considered humanoids. Most undead are controlled by the Scourge.
Some creatures are not classified as any of those above creature types. They have their own properties or differences and therefore can't be placed in a category. Examples include the naaru, Enchanted Tiki and inanimate objects that act as vendors or trainers (such as vending machines and Scryer bookcases).
Having no category, they are not affected by abilities that target a particular type of creature, such as [Sap] or [Polymorph].
Prior to Patch 4.0.1, uncategorized creatures had "Unknown" as a creature type; since then, the type is omitted from the tooltip.
Over the years that World of Warcraft has been active, many creatures that once had no type, have been slowly moved into the above due to certain mechanics. Several creatures were moved to the newly created type, Aberration when it was added in patch 5.1.0.