It is a creature's soul that determines if they have the possibility to pass on to Heaven and become angels.
It can also be used to describe someone who was mortal and has now died and passed on. Especially devils and people who have been damned to a personal hell are often refered to as 'souls', but the term includes angels as well.
Creatures with souls[]
Usually sentient creatures from the mortal worlds have souls, while animals, for example, have not. A soul is what you become when you have died. It is basically a second physical form that allows you to cross over to the 1st and 2nd plane of the universe where Heaven and Hell is when you die rather than disappearing forever.
Creatures that don't hail from a mortal world usually don't have souls. This includes demons and cherubim among other types of beings. A soul is an inherited trait, however, which makes it possible for a demon and a mortal to have offspring that will have a soul.
Physique[]
A soul is the ideal version of the person who has died. That means that all illnesses, injuries, deformities and other handicaps that a person might have had as a mortal will be gone. A soul doesn't have the same physical needs as a mortal and can technically exist without eating, breathing or anything else mortals have to do to stay alive. The body still functions like that of a mortal , though, and especially breathing is a hard habit to get rid of. A soul isn't imune to injuries either and can get hurt and ultimately die just like a mortal.
Souls don't age, but can change what age they appear to be at will, although it is just as slow a process as when a mortal ages. Most people do this unconciously and end up looking like the age they feel they are.
Souls are not capable of having children.