The undead are at once pitiable and utterly horrific. It is very seldom that any creature voluntarily becomes undead, and those few that do - usually in a misguided attempt to achieve immortality - soon come to regret it. The existence of an undead creature is one of unrelenting torture which always drives it into madness, if it has any mental faculties at all remaining.
The lowest in the hierarchy of the undead are the zombies and skeletons. Some skeletons are simply magically animated automata, and are not truly undead as they retain no connection to the spirit of the original owner of the bones. They could just as easily be a bunch of sticks and would still work well enough, but a skeleton has a certain morale effect which a scarecrow - even an animated one - does not. Zombies, on the other hand, have the spirit of the creature still trapped within the slowly decomposing corpse. They have just enough self-awareness left to know that they are dead, and they desire life more than anything. This is usually translated by the unfortunate creature as a physical hunger . . . they try to obtain life for themselves by eating things which are alive, and since they sort of remember being people they eat people. Simple really.
Zombies are not really very useful as servants; they smell appalling for one thing, they start falling to bits and leaving slime-trails after a while, and they must be firmly controlled to keep them from trying to snack on any of the living about the place. Though they are easily dominated, having little ego remaining, it is difficult to keep them to a task if anyone with any mind-control powers tries to distract them. They are constantly cold - the chill of the dead and all that - and are attracted to the warmth of a fire. If left to their own devices, zombies will happily walk into the middle of a raging bonfire in an attempt to get warm again; this trait makes them less useful than they might otherwise be on the battlefield if the foe has time to prepare a warm reception for them. (Ho ho, that's a joke, get it?)
As a zombie ages, it loses its flesh and the last remnants of its ego. The resident spirit is usually completely insane by the time the fleshy zombie is transformed into a boney skeleton, and a skilled necromancer (or psychotherapist) can manipulate it during the transition period into an obsessive compulsion to obey him or her. This process is not without its dangers; the obsession can easily become a homicidal compulsion if the manipulator is careless or inexperienced. If successful though, the skeleton becomes a terrifyingly obedient slave to its master, and while requiring a little maintenance to keep its sinews from drying out and shrinking, the skeleton is a very useful servant indeed. A properly imprinted skeleton is very difficult for an unauthorized person to sway from its allotted task, and doesn't have the body-odour problem of a zombie. And what's more, they can be painted in attractive shades to match the necromancer's decor. Skeletons retain the life-hunger and desire for warmth of their earlier form, but if soundly conditioned will keep their lusts in check at the orders of the master.
The enchantment which keeps these two forms going is applied to the skeletal framework of the body. The flesh then becomes surplus baggage; it is, after all, just a hundred or more pounds of dead meat. For this reason, zombies are very slow and sluggish in their movements, and though they don't actually get tired, each step and movement is a great effort for them. Once the flesh is gone, the animated skeleton becomes quite nimble. It magically retains the physical strength of its living form, but has considerably less weight to carry around. It is common for necromancers to leave their new zombies in rat-infested dungeons for a while to strip them clean, but this can have a deleterious effect on the imprinting of the resulting skeleton since the process ideally requires constant contact with the wretch during its slide into insanity. If the process of being eaten alive (or un-alive) pushes the zombie too deeply into psychosis before the imprinting can properly begin, the resulting monster becomes useless as a slave and must be destroyed or discarded. Creating these undead servants is not generally regarded as the act of a nice person.
Zombies and skeletons do not perceive the real world in the same way as living creatures. They are very sensitive to temperature, and posses a sort of infra-red "vision", but their most important sense as far as their masters are usually concerned is an ability to detect the presence of living creatures regardless of whether or not they are actually visible, or for that matter, material. This "life-sense", coupled with the fact that they never sleep and never get bored, makes them excellent guards. They are quite difficult to disable since they don't bleed or go into shock, and in fact the only way to finally destroy a zombie or skeleton is to burn it to ash and scatter the ashes. They can be neutralized by breaking them apart so that they are unable to move, but unless they are completely obliterated the spirit stays with the bones and the skeleton can later be reassembled, as good as new.
These lowest undead have no real discernment or initiative of their own except when it comes to feeding their hungers. This means that they must be instructed carefully and literally; they will carry out instructions to the letter, no more and no less. The complexity of the instructions successfully given them can be quite high as long as they are precise and logical; if a later instruction opposes an earlier one which has not been cancelled, the creature will do nothing at all. Whether or not obedience to detailed instruction can be defined as "learning" is a matter for philosophical debate. In any case, skeletons can not extrapolate from previous experience and are incapable of adjusting to an unforeseen change in circumstances. They do keep a limited memory of conditions, people and objects with which they were familiar in life, so not every single thing in an instructional chain needs to be closely defined as long as it is recognizable to the skeleton. For example, the zombie of a warrior will recognize a sword and will understand the concept "fighting", and will probably remember basic combat technique so that it need not have every move and counter-move spelled out for it.
Ghouls, the human or humanoid creatures who haunt graveyards and feed on the flesh of corpses, are traditionally assumed to be undead but actually they are not. They are simply a rather unsavoury variety of anthropophage. They tend to look, and more importantly, smell quite similar to zombies, and some which are infected with a disease from the brains of the dead are called "giggling zombies" because of this resemblance and the constant involuntary chortling which they produce. A true zombie is unlikely to notice a dead body, let alone eat one, but like zombies, ghouls will quite happily devour living human flesh (if they can get it without too much risk to themselves.)
The word "lich" means a corpse; a lichfield is a graveyard. In the context of the undead, a lich is someone, usually a wizard, who has found a way to stave off death without achieving true immortality. A lich is obviously undead (unless disguised by illusion), showing a corpse-like visage and exuding an aroma of putrescence. The lich suffers in much the same way as a zombie, but unlike zombie-hood the process of becoming a lich is pursued voluntarily and is assumed before death. The intelligence and memory are not immediately impaired by it, though the sanity of someone who does that to themselves on purpose must be dubious, to say the least. Apart from averting the complete onset of death, lichdom does not confer any special powers.
Of all the undead, it is the vampire which provokes the most fear. They are strong, charismatic, utterly unscrupulous and often quite wealthy. These are, it is true, horrific attributes, but the thing about vampires which really terrifies people is the idea that they can sneak up on you while you're asleep and drain you dry of your life's blood, and there's not a thing that you can do about it. (Just like tax-collectors). Of course, vampires are not unstoppable; they have been destroyed in the past and no doubt will be in the future, but any bloodsucker who has been around for any length of time must have one or two clues about the basic techniques of survival. The folklore about vampires differs from culture to culture, and in fact no two vampires are exactly alike. Some really are allergic to sunlight and hate garlic, but others are quite happy to spend their days sunbathing and eating Italian food to build up their strength for the night's hunting. The only thing that you can be sure of when vampire-hunting is that the Creature Of The Night could be anyone. Well, almost anyone, I mean it couldn't be you . . . could it?
If caught in the unenviable position of having to deal with a blood-crazed vampire, remember that most of the folklore contains a kernel of truth. So shove that sharpened oak stake right into his chest - who knows, maybe it will work. And if you're that close with nothing better in your hands than a pointed stick, what have you got to lose? Try anything; throw garlic, holy water, mirrors, squeal holy phrases, anything which might have any chance at all of working.
Don't count on becoming a vampire just because you've been killed by one; it doesn't work that way, and if you are snuffed by Count Yorga or his cronies you are DEAD DEAD DEAD. The process of being sucked dry by the Living Dead does something really unpleasant to the spirit or soul or whatever, and even if the resulting corpse is magically revived it will never be anything more than a vegetable. A vampire has to make a conscious decision to create a new vampire, and it involves some unspeakable vampire stuff about which Mortals Are not Meant To Know.
Vampires don't seem to suffer as badly from undeadness as the other types. They must have fresh blood daily, preferably from a human or similar but they can get by on animals if they have to. If they fail to get their daily dose, existence becomes more and more uncomfortable, but at a pinch they can make do with as little as a litre or so a day (they need more if it comes from an unintelligent beast) and so can keep victims around for a while before they become too anĉmic to be worth tapping.
The Living Dead are not especially sociable. Other vampires are not only competitors for the available food-supply, but the more there are in an area, the more likely it is that some do-gooding busybody will smell a bat. Nevertheless, from time to time even vampires become lonely (or tired of doing their own housework,) and then they might create lesser vampires to be their companions and/or servants. The process is not particularly difficult, and there is no limit to the number of new vampires which can be created, but it is very rare that more than three or four lesser vampires will be in attendance at a time. Lesser vampires share the characteristics of their master, but they are always weaker in every way and are therefore more easily dealt with. Snuffing these wimpires can provide valuable information on how to deal with the Big Guy, but be warned that he (or she) will almost certainly be more dangerous than all of the little ones combined.
These creatures are the incorporeal remains of some dead intelligent creature, usually humanoid. They are not necessarily malign spirits, but it pays to treat them with extreme caution until they are positively shown to be harmless. They are formed when a person dies or is killed untimely, with some unfinished business to attend to. Perhaps they have to take revenge on a murderer, in which case they are known as revenants, or maybe they have to atone for some really nasty business they were involved in while they were alive. Whatever the reason, they can always be laid to rest by performing the required act for them (though you may not want to, especially if it's something really disgusting or tiresome). Determining what it is that they want you to do can be problematic; often they aren't allowed to say directly and you are left in the absurd position of having to play "Twenty Questions" with a boogy-monster to get the information. Complications arise when it is no longer possible to create the right conditions for their release - if, for example, the murderer on whom revenge must be taken has already died peacefully in his sleep of extreme old age, having lived a full and enjoyable life and died wealthy and respected by everyone. In this case the only way to remove a haunting is to destroy the haunting spirit. This is hardly fair on the ghosty concerned, since it might not be its own fault that it has been condemned to walk the earth, but after all we can't have spooks hanging about the place and frightening people to death. And if the regrettable task of consigning a bogey to the void should fall to you, you can take comfort in the thought that it's probably completely loony by now and the obliteration is more like euthanasia than anything else. Of course, the ghost is not going to go quietly and is quite difficult to damage unless you can fight it on the ethereal plane. . . but it can probably affect you easily enough in your own world. So take care.
Tombs the world over are the lairs of these undead. Mummies are corpses which have been preserved by embalming, while the bodies inhabited by barrow-wights are preserved or reconstituted by magic. In both cases, the corpse is possessed by a spirit, usually malicious, for some purpose. Sometimes the spirit is summoned to defend the tomb from interlopers, but often the possession is initiated by an evil spirit which requires a host in the material world for some reason. Occasionally the spirit is that of the original owner of the body, but the motivations of the wight may well be quite different to those of the living person. They seldom venture out of their tombs, and when they do it is by night, but they can often extend their powers out beyond their immediate confines to affect those outside. Wights and mummies have an evil reputation, and should never be taken lightly. They are invariably very strong, both physically and psychically, and barrow-wights posses terrible necromantic powers of control and domination.