I'm kind of annoyed by monster manuals. Certain things are cool, like making up new monsters and the like, but some things are annoying. It comes with the territory of course; but the fact there needs to be slightly different rules and write ups for virtually identical creatures, or basically inventing new creatures based on commonly used words to refer to many at once, is just a sort of normal tabletop thing. It's not even necessarily bad, it just agitates me a bit.
No where more is this obvious, at least to me, then undead. Like I get the difference between zombies, skeletons, ghosts, and vampires. Things like that. But then you ghouls, wights, mummies, wraiths, ghasts, phantoms, all kinds of silly shit. In D&D, I'm pretty sure all of these are write ups for different creatures that fall under the class of undead being. Once again, I understand why this is done; it's to create more varied monsters to encounter, and undead being an important type of monster its important to keep all of these as unique entities of varying power levels for use in games.
But what about simplicity for the sake of simplicity? Here's some equally pointless reductionism.
Ghouls are the most basic type of undead, and usually result from a corpse being disturbed or buried improperly. They are unliving corpses, animated by sheer malevolence. They tend to be very hungry, living out things they did in life stupidly. Skeletons and zombies are incomplete or weak Ghouls; draining more blood and life force will make them more whole. Maybe really powerful ones explain vampires or liches or something; they just ate their fill and have regained mortal cleverness.
Wraiths are ghostly entities, more like spirits. They may be dead who were cremated, or a dead person missing body parts; returning their body parts to their corpse and resting place may be enough to put them down. They aren't totally physical, and as such can only be hurt by magic weapons or spells. They are usually made of grave dust and always appear as that generic hooded figure floating around.
Ghosts are a separate thing. They aren't undead and typically can't manifest enough to hurt anyone. They're just the spirit of someone who died and hasn't like “passed on” yet.
Death = Sleep
When people die, their corpse shouldn't be disturbed. The best “rest” a dead person gets is when their body is entombed around family members, with treasures of their life, in a sanctified tomb, and so on. The less of these qualities that they have, the more “unrest” they get. Highway bandits killing a person and leaving their corpse for the birds is very likely to create a very unhappy corpse, and that corpse is likely to get up and start haunting people. Sometimes this creates a revenant, an undead obsessed with killing those who killed it.
More commonly though, undead are created when people disturb their tombs, or they weren't buried in the first place. Raising zombies and skeleton minions through necromancy is more just about disturbed and defiling tombs, and then binding the spirits into your service once they wake up; hence the whole “zombies secretly hate the necromancer and will kill them the moment their chains are broken” sort of trope we see a lot in fantasy media.
It should also be noted that more powerful beings leave more powerful corpses, hence they're more likely to get up if not placated. Of course, the corpse could be the actual person just sleeping in the ground, or a different “soul” if you subscribe to that theory. This is why ancient Kings and Heroes are always buried with tons of treasure and around traps and stuff; otherwise you'll have a powerful spirit on your hand.