Tales of Tolgard
Incorporeal basically means not solid. In regards to creatures, it means that they do not have a solid form. It might mean that they are gaseous in nature or they do not have a body per se or that the majority of their essence is on another plane. Whatever the reason, these creatures are difficult to hit and can move through solid objects at will. Some creatures have in-corporeality as an attribute, and many of those are either undead or from the Plane of Air.