A Treatise on Undead
Because there are so many misconceptions about undead, it is helpful to start by clarifying some basic facts about these creatures in general.
Undead and Negative Energy
The term "Undead" identifies creatures whose normal biological processes have ceased, yet which continue to exist through the application of magical energy. Importantly, they differ from constructs or other entities who function via magic but were never "alive" in the traditional sense in the first place.
All creatures require energy to function, and the multiverse produces this energy from two sources -- the Positive and Negative Material planes. The natural life energy which normal creatures depend on is, we are told by philosophers, analogous to Positive magical energy, and while mortal creatures can generate this energy naturally, they can only do so for a certain amount of time. Gradually, this ability is lost through the process we call ageing, until ultimately the creature cannot produce enough to function. Normally at this point, the physical body dies, and the soul or spirit passes on.
There is, however, an alternative, and that is to harness the energy of death itself and substitute Negative energy for Positive in order to continue existing. Negative energy works differently than Positive; most significantly, it doesn't fade over time and can actually become stronger, and it more readily replenishes itself naturally. While the full effects of existing on Negative energy will be discussed below, the fundamental point is that it offers immortality in the form of unending existence without deterioration or breakdown.
Yet to be clear, while undead go on "existing", they certainly do not go on living. The change from Positive to Negative energy affects more than just the physical body, it surely affects, damns, and perhaps ultimately destroys whatever spirit, soul, or essence that creature once had. The characteristics that made them human (or elven, or dwarven, or orcish or whatever), the thoughts, needs and emotions of their previous life are gone, and replaced with an irreversible Evil that drives their continued existence. It further fuels them with an insatiable hatred of all that lives and a desire to bring darkness where there is otherwise light. It isn't even un-life, it is truly anti-life, and of all the evils of the world, none is so clearly identified as the power of Negative energy.
A brief mention must be made of the one apparent exception to this rule: mummies. Several religions, most notably Egyptian and ancient Sumerian, famously possess rituals for creating mummies, and all feature the use of Positive rather than Negative magical energy in their creation. The complexities of this process are part of their appropriate religious doctrines and too complicated to explore here, but what can result is an undead creature that isn't powered by negative energy and isn't inherently evil. While mummies share many standard characteristics with other undead, there are notable differences, and these are discussed in the description of the specific creature.
Types of Undead
Undead are generally classified in three broad categories: corporeal, semi-corporeal, and non-corporeal. Corporeal creatures have physical bodies, and include zombies, skeletons, ghouls, ghasts, wights, mummies, and vampires. Semi-corporeal undead have no physical bodies and consist entirely of magical energy, but they interact with physical objects as if they were physical themselves -- they can wear clothes or carry items, but can't walk through a wall. Examples are haunts, reavers, and wraith lords. Non-corporeal creatures have no physical bodies, and don't interact with physical objects. They include shadows, spectres, ghosts, and wraiths. These creatures are only affected by magic, whether as spell, in a weapon, or other forms.
There are a couple of special cases. The lich is one; most are corporeal beings but their magic is strong enough to transcend their physical nature, and function more like semi-corporeal creatures. The strongest become essentially non-corporeal for all intents and purposes. The Death Knight is another special case because of its demonic aspects, and some expects considered it "undead related" rather than a true undead type. Here we follow the traditional classification and include it as undead.
Typical Undead Characteristics
While there are a wide variety of undead types, all share a number of characteristics due to their nature. This is primarily a function of their use of negative energy, and while mummies are an exception to that, they still seem to share most of these conditions.
Because they are not living, most undead cannot be "killed" in a normal sense, nor are they affected by pain. You can't stab one in the heart, and most don't care if you cut off an arm or a leg. Most can be physically destroyed in some way, whether chopped to pieces or magically blown to bits; but even then the destruction of a physical form, or dissipation of an incorporeal one, may or may not mean the actual and permanent destruction of the creature in question. Many higher undead types will, unless killed spiritually as well as physically, re-manifest themselves over (usually a long) time. It is for this reason that areas known to be haunted by undead often appear to re-populate themselves over time or repeated visitations. This, of course, does not mean the creatures cannot be permanently destroyed, only that it often requires more than just a physical element.
In a similar vein, since they are already dead, there are a number of things that simply don't affect them. Toxic effects like poisons or diseases have no effect on undead creatures, nor do gases or a lack of air, as they don't actually breathe. Although most have natural senses, they aren't dependent on sight and sound as we are, and can generally function in the absence of either. And finally, it's no joke that being already dead makes you hard to kill. Most undead must be damaged well beyond what the normal mortal form can sustain, in many cases quite literally hacked to pieces.
The Negative energy that sustains (most) undead creatures makes them naturally magical themselves, and has common implications. Virtually all undead, save for zombies and the simplest skeletons, can be damaged only with magical, blessed, or silver-alloyed weapons, and in some cases only ones of exceptional quality. Most are resistant to elemental magic, like fire or lightning, and some channel enough Negative energy to gain outright Magic Resistance. None are affected by sleep, charm, or mind-control magic, and none feel either fear or pain. Undead don't sleep or feel fatigue, and spellcasters can channel high-level spell energies without the draining effects this has on mortal casters. And while some undead types appear to "feed" (like vampires and blood), they don't actually need to eat to survive, and will simply go dormant if their requirements aren't met.
The ability of almost all undead to drain energy from their victims, whether through attribute damage or life energy itself, is another direct result of their connection to the Negative plane. When they contact the living, the positive and negative forces cause mutual cancellation, but the natural flow of Negative energy replenishes itself so much faster that in effect, only the living creature is actually damaged. If the contact continues long enough, the negative force will override the positive completely, resulting in death, or even worse, the victim becoming undead themselves. Most frightening, most draining effects don't heal naturally, but require powerful clerical magic to restore a victim to normal. Worse still, death through life energy drain is absolutely irreversible through any means short of a Resurrection spell -- if even possible at all. There are many cases where level drain is associated with the full and complete consumption of the spirit or soul involved, and in these cases, there is no power known that can bring them back.
Thankfully, there are at least a few things that Negative energy use leaves one vulnerable to. Holy magic is the most fundamental (not even all evil deities approve of undead), providing a wide variety of spells specifically attuned to combating Negative power as well as the well-known power of Turning. Good-aligned holy powers in particular are also capable of producing another universal anathema to undead in the form of holy water. Arcane casters possess certain shells and barrier effects which can inhibit or completely prevent the passage of magical creatures in general. And perhaps the most well-known power of all is natural sunlight -- repellant to all undead and outright deadly to many.
It is finally the nature of Negative energy that, with rare exceptions, those who depend upon it are inevitably made irreversibly Evil by their choice. As Negative energy is the essential opposite of the natural force of life, so do those that depend upon it inevitably develop a hatred for and become the enemy of all living things. Some, perhaps, are less violent than others, and some rare Haunts and Ghosts may seem almost Neutral in their nature; but in the end, it is a power incompatible with the principles of life and which cannot exist alongside it. Cultures and religions can and often do disagree on the definition and limits of what they call Good, but the power of un-life is one universally agreed upon as the very embodiment of Evil itself.
Turning Undead
Because an undead existence is anathema to virtually all religions, most clerics have a natural ability to focus religious power against undead. As this effect is typically used to repel undead creatures, it is commonly called "turning", although in the case of some clerics of some evil deities, the identical power is used to command them instead. Each religion has its own specific ritual, essentially a magical spell though it requires no expenditure of power. While it is an ability even the novice practitioner can use, it does typically require a physical focus, usually a previously-dedicated totem or symbolic item dedicated to the cleric's deity or pantheon.
Turning is a powerful effect, but it has limitations. It is a focused power, the cleric can't generally move while channeling and it generally has a maximum range of between 30 - 60 feet. There are some undead that aren't affected by turning at all, and those of higher level than the cleric are much harder to affect. The power tends to work best against groups of undead of equal or lower level than the turner; the greater the difference, the more likely the success, and the more undead likely to be affected.
Undead to be turned have to be confronted directly, and the turn has to be "aimed" at them -- an undead sneaking up from behind would not be affected if the cleric isn't aware of it. Generally, undead that are Turned cannot approach the cleric, nor attack them, and in fact must retreat out of range or line or sight if possible. They can return, but not immediately, and they can be turned repeatedly, but not infinitely. Repeated turn attempts on the same undead tend to become more difficult, and ultimately lose effectiveness. Similarly, undead who have no ability to retreat (already backed into a corner, etc.) may be unaffected by an otherwise proper turn attempt.
The converse of turning is controlling. The ability to control-turn is not necessarily an automatic property of all evil clerics. The cleric must further be of a religion or deity with specific connections to the underworld or undead power. It is also a dangerous proposition, as free-willed undead may resent and react to control attempts. However, for clerics of appropriately dedicated deities, even powerful undead can be commanded to the cleric's bidding.
It should also be noted that while clerics can turn undead, the opposite is also potentially true -- a cleric confronted with an exceptionally high level undead can find him or herself rendered near helpless by the creature's raw power, or, if evil, commanded by it.