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.
All creatures require energy to function, energy normally provided by the biological process of life. Undead creatures are dead; they no longer produce life energy naturally and in order to exist they must get that energy from somewhere else. In the simplest sense, undead creatures substitute magic for biology, and in almost all cases, this means energy from the Negative Material Plane.
Magical power has, of course, two fundamental point of origin -- the Positive and Negative Material Planes. The natural life energy which normal creatures depend on is (we are told by magic 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. When a creature is no longer able to maintain life (positive) energy, its alternative is to sustain existence through the opposite energy form -- negative energy, a power which originates from the Negative Material Plane.
Thus all undead types, with the exception of Mummies (see below), maintain some kind of connection to the Negative Material Plane in order to provide life-sustaining energy. While some undead do physically "feed", and may need to do so in order to function effectively, it is still the connection to the Negative plane which gives them the power to exist. The more powerful the undead type, the stronger its connection will be.
Types of Undead
Undead are generally classified in two broad categories: corporeal, and non-corporeal. Corporeal creatures have physical bodies, and include zombies, skeletons, ghouls, ghasts, wights, mummies, and vampires. Non-corporeal undead have no natural physical form, though some can choose to take on or take over physical bodies. Shadows, spirits, haunts, spectres, ghosts, reavers, and wraiths are all non-corporeal undead. The Lich is considered a special case; while most typically have a physical body, many have enough magical energy to transcend their physical nature. The Death Knight is another special case because of its demonic aspect, and is considered by some "undead related" rather than a true undead type; we will discuss them here as a normal undead type.
Typical Undead Characteristics
Undead have a number of common characteristics. Because they are not living, most undead cannot be "killed" in a normal sense. Most can be physically destroyed in some way, whether hacked to pieces or magically blown to bits; but even then the destruction of a physical form, or dissipation of an incorporeal form, 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 usually requires more than just a physical element.
Any creature which draws energy from the negative material plane, even to a small extent, becomes by definition a magical creatures, and most (exceptions noted below) are only affected by magical, blessed, or silver-alloy weapons. Certain forms of magic are almost universally useless against undead, notably sleep, charm, and mind-control effects. Element-based effects (fire, cold, etc.) will tend to do half damage to undead creatures, and poisons have no effect whatsoever. Conversely, they do become vulnerable to certain positive-energy or holy effects, including clerical turning (see below), and all negative-energy creatures (even incorporeal ones) are damaged by holy water, which tends to act like an acid. Natural sunlight is also a repellant to many undead types, in some cases a deadly one.
The negative energy which gives undead existence is also the source for the most dreaded of all undead abilities, the draining of an individual's very life force itself. Negative energy is, in a sense, the opposite of life force, or positive energy. By being permanently connected to negative energy undead creatures naturally degrade the life force of living things they come into contact with. This power is exceptionally deadly for two reasons. First is the near-permanence of the effect. Life energy drained does not heal with time or with most magical healings, but only from specific and powerful dedicated magical spells available only to high level clerics. The second danger of life drain is that a character killed by it is unrecoverable by anything short of a god-generated Resurrection spell, and thus represents as permanent a death as is conceivable.
Finally, the association with negative energy causes undead to be almost universally evil in alignment. This is again a reflection of negative energy as the essential opposite of life-energy, and as a general rule, all undead forms possess a hatred of living creatures and will seek to destroy them. There are a few exceptions, notably shadows and some rare ghosts, and in theory it's possible a Lich could be purely neutral in alignment (good luck with that).
Turning Undead
Because an undead existence is an anathema to virtually all religions, most clerics have a natural ability to focus religious power against undead. As this power is typically used to repel undead creatures, it is commonly called "turning", although in the vase of some evil clerics, 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. It does typically require a physical focus, usually a blessed holy symbol (often made of silver or other precious metals) or a dedicated totem of some kind.
Turning is a powerful effect, but it has significant limitations. 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. The number of undead which can be affected also varies substantially, based most obviously on the level differences between cleric and creature.
Generally, undead that are Turned are forced to retreat from the cleric, but only for the moment, and they will be able to come back. Undead creatures 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.
In certain cases, particularly when the cleric is substantially more powerful than the undead he or she confronts, undead are not turned but are actually overwhelmed and destroyed by the cleric's display of power.
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.