The following post is by Yobgod from OYT:
I've mentioned the basis of this idea in other posts, but this seems like a suitable place to flesh it out a little better (except the skeletons, of course).
As a theory, it's applicable to non-D&D systems, but I'll focus on the D&D interpretation here.
There are seven main "types" of undead, based on the retention or loss of the three primary qualities of the original creature, those being:
Body - The corporeal form, possibly decayed to a greater or lesser extent. (Strength, Dexterity)
Mind - The capacity for independent thought and reasoning. (intelligence, Wisdom)
Spirit - The sense of self, memory. (Charisma)
Most of the undead one may encounter are powered by negative (unlife) rather than positive (life) energy, although the latter are theoretically possible and would tend to be less corrupted by the hatred for life that our standard undead possess. In any case, this leads us to the following forms of undead:
The Animated Corpse (Body only)
Examples: Skeletons, Zombies and everything in between
Description: These common pests are merely the shells of the deceased made to move again by the energy of unlife. They lack both a sense of self and the capability for reasoning. When created by a Necromancer or Cleric they can be made to follow simple instructions, otherwise they simply move towards and attempt to destroy living creatures. Skeletons require more magical energy to animate, since more physical form must be compensated for with magic, but as a result tend to be much nimbler than the shuffling zombie.
The Hungry Corpse (Body and Mind)
Examples: Ghouls, Ghasts, Mohrg, Wights* and similar creatures
Description: These corporeal undead can be significantly more dangerous, as they retain their intelligence and cunning. Lacking a sense of self, however, they are completely driven by whatever hungers the hateful negative energy powering them has focused on. They are crafty and ruthless and remember nothing of life, seeking only to kill and to slake their insatiable hunger.
The Corrupted Corpse (Body and Spirit)
Examples: Trap Haunts, certain Tomb Guardians, Wights*
Description: These sad creatures retain their form and retain a sense of who they were, but have lost the capacity for independent thought. Often they endlessly relive their last moments or carry out their last commands, interpreting the present through both the distorted lens of time and the corruption of unlife. Unfortunate or not, they can still be extremely dangerous. Appeals to the emotions of their past may help, but appeals to reason will fail, and evidence of their current state is likely to drive them into a killing rage.
Sidebar: Wights - There have been several different creatures known as Wights, some of which are clearly Hungry Dead (3.5 MM Wight) while others fall more clearly into the Corrupted Dead classification (Tolkien Barrow Wights). Confuse them at your own peril.
The Corrupted Spirit (Spirit only)
Examples: Haunts, most Ghosts
Description: Similar to The Corrupted Corpse (and actually more common), these undead retain a memory of their identity and emotion, but have lost both their body and mind. Incorporeal, they are often tied to a particular place or longing. While some are relatively harmless, the negative energy that animates them makes their touch anathema to the living, whether they intend harm or not. Sometimes these are corrupted corpses whose corporeal forms have worn away to nothing.
The Hungry Spirit (Mind only)
Examples: Shadows, Wraiths
Description: Nothing but an evil will, fueled by hunger and hate for life, hungry spirits can, like their corporeal brethren, be cunning and manipulative.
The Lost Soul (Spirit and Mind)
Examples: None I know in D&D.
Description: This is the essence of a person stripped of their corporeal shell, corrupted by negative energy and left to roam the material planes. Having both thought and memory, unless they were particularly evil beings to start with, they are likely to be somewhat tortured by their current state and prone to fits of both rage and sadness. Sometimes, an evil creature will become a Lost Soul in order to bind themselves into an object or other prepared form in order to approximate eternal life. Sometimes an even more evil creature will use similar techniques to bind an unwilling soul into an object or form.
The Unliving (Body, Mind, Spirit)
Examples: Vampires, Liches, Death Knights
Description: These evil creatures voluntarily embrace undeath through one means or another and thus retain their form (often augmented) and facilities. The conversion to run on clean burning coal negative energy often takes it's toll however, and slowly corrupts the mind of all but the strongest of wills. Those wills that are already wholly evil in nature have no problen with this.
It is theorized (depending on your setting), that there can also be alternate forms of undead that remain powered by positive energy, despite no longer having a functioning metabolism. There are several of these (which are generally more suited to PCs). In some cultures Mummies are believed to fall into this category, may have any combination of Mind, Spirit, or both, and are left as tomb guardians. In other cultures they are fueled by evil like any other undead. Caution, as always, is advised. Other examples include Graveborn, Revenants, Petitioners and all other cases where thought and motion is maintained by magic and positive energy rather than normal bodily function. Some would seek to lump Warforged into this category, but they are different because they do rely on a functional metabolism... an artificially created one, but a metabolism none the less.
The positive energy fueled combination of Mind and Spirit is likely to be what we usually refer to as "the soul", which normally travels on to the outer planes once the body ceases to function, although religious representatives have declined to comment on this. This explains why a creature who has been converted into any undead with Mind or Spirit is unable to be raised from the dead or move on to their final reward/punishment. Many, however, believe that even having your corpse raised as a simple Zombie or Skeleton is sufficient to prevent the soul from moving on or coming back. It may be that the soul maintains an attachment to it's physical remains and that the infusion of those remains with negative energy taints the soul as well through that link.
If positive-energy undead exist, some form of unliving is the most appropriate option for an undead PC. Any form of mindless undead is obviously unsuitable for a PC, although you could play one with no spirit as a free-willed undead who has merely lost their memory and general sense of self. (Maybe it was stolen and they are questing for it?)