Creature

A creature, in terms of game mechanisms, could be any object with a non-depleted Health bar.

Creatures are typically classified as either an ally or a foe. However, some special NPCs (usually service-providing NPCs) are considered neither -- they are not valid targets for skills that target either ally or foe. Those "neutral" NPCs can still be damaged by AOE skills not targetted at them though.

Health depletion
Quite naturally, only corporeal creatures leave a corpse behind them (not all of them are exploitable though). The death of any non-Spirit creature will trigger the effects of Soul Reaping, provided that conditions are met for the energy gain.