All life within the Universe generally falls within 1 of 3 catagories: Mortal, Immortal, and Ethereal.
The vast majority of life that exists in the Lower Planes are mortal, in that they live for finite periods of time upon which their physical bodies expire. Upon shedding their bodies, their spirits are returned to a mysterious and ill-defined concept called the Cycle through which they are reincarnated in new bodies in order to continue on the path toward enlightenment. However, some spirits end up weighed down by the evil they committed during their life and are dragged to Hell, where the weight of their sin or negative karma is burned off until they can rejoin the Cycle. In Hell, Mortal beings are called demons. While some evolved there naturally, most of the intelligent variety are the spawn of the first Devils, who were the Immortal children of the Fallen Celestial known as Satan.
Immortal is the highest state available to Material Beings. Those who have achieved immortality no longer age and are immune to all sickness and disease. As Immortals they no longer need to eat, drink, breathe, or sleep, though many continue to do so out of habit. They are very difficult to injure, and when they are injured most are able to instantly regenerate any damage. Against a sufficiently powerful opponent or force however, even an Immortal can be destroyed. While some Immortal beings are born that way, like the Gods, others achieve it either through great effort on the path toward Enlightenment or artificially through use of science or magic like the Watchers, Omnians and Vampires. In Hell, Immortal beings are called Devils. These can either be born into that status, such as those who are the direct children of the first Devil, Satan, or can be made when a demon manages to attain a certain level of power. It is generally believed that achieving Immortality is one of the steps of Enlightenment, as any who ascended to the status of Ethereal were once Immortal.
Ethereal is the highest state of life, the state necessary to exist in Heaven and be close to the Source. Ethereal beings exist in a state of pure thought and energy, and are vastly powerful to the point of practical omnipotence. They do not recognize linear time, and are commonly able to exist in multiple times and places at once. The most common Ethereal beings are Celestials, who are either created directly by the Source or are the result of a Material Being achieving Enlightenment. While Celestials are the best known and seemingly most common form of Ethereal life, they are not alone at this status. The Guardian of the Midplane, Adam, has stated that there are creatures which were born from the Source’s nightmares that are not always friendly to Material Beings. These Outer Beasts, as he calls them, usually despise matter and exist far away from Creation in the Void. Adam describes these creatures as fearsome beasts that will attack any being that approaches them, and that they and the Celestials generally avoid eachother.