I have always had a rather clear idea of how I wanted undead in my games and what their origin would be. This ties fairly closely with Clerics, religion and has a bit of Dante's Inferno in it as well:
The Dead - Dead which are both faithful and buried on consecrated ground will never rise from the dead. If the consecrated ground is somehow marred or tainted, this is no longer true. The dead which are not buried on consecrated ground are vulnerable. The dead of the sinful are likewise vulnerable, although consecrated ground makes it significantly less likely they would rise. The sinful who are not buried on consecrated ground are very vulnerable, and this chance goes up the more horrible their crimes. Lastly, those murdered or slay themselves are also vulnerable.
So, clerics, priest and their acolytes definitely make it a priority to firstly consecrate holy ground and bury their dead therein. Otherwise bad things can happen, especially if you have some real bastards among your population. Site of mass battles are especially vulnerable considering the amount of violence was wrought and the likelihood of lost or forgotten bodies - or simply mass graves or otherwise desecrated remains. All a prime site of dead rising.
Consecrated ground, just like consecrated water, can harm undead. Most will not cross consecrated soil, but some of the exceedingly powerful undead can make a save vs Death to do so. Consecrated ground, even a handful, will harm undead if they touch it, 1hp per round. A Cleric's weapon can likewise be consecrated by himself to deal 1 extra point of damage to undead. Consecrated holy water is even more potent (as holy water rules). Consecrated relics are more potent still, especially in the hands of the person who consecrated it (turn undead rules).
Now for the actual undead:
Zombies / Skeletons - Skeletons and Zombies are more or less identical, but in different stages of decay. Zombies are Skeletons are the unwilling undead. That is, not particularly evil individual that have been made unto walking undead to serve a higher (evil) will. The bodies are unwilling, and so are forced into action that is not of their nature - and so their awkward and lumbering movements appear forced and strained. These undead are soulless vessels however will obey their master's will without question. (ie, slow zombies)
All undead after this point forward are the sinful dead and considerable more dangerous. Unlike the unwilling dead above, they do not hold back, and move quickly with deliberate determination (ie, fast zombies). These undead do have souls; often twisted, evil and malicious.
Ghoul - Ghouls are the remains of carnal malefactors. Those who went to extremes of lust or gluttony and now seek out the most perverse pleasures or to eat the most decrepit of things (with peoples on top of their list). They usually appear as they died - a corpse in the beginning stages of decay, smelling of rot, dirt stained with perhaps a seeping wound or noose around the neck.
Wight - Wights are the remains of the extremely avaricious or miserly (including many previous holy-men). Wights are distorted versions of their previous lives. They often have overly large hands, tongues or skin that is sloughing off of their body. All Wights drag around huge sack of what which they desire - usually heavy coins but sometimes other things such as small children. Wights attack viciously those who have more of what they desire (wealth, belongings, youth, beauty and so forth). They will ignore those who appear to have nothing that they want.
Shadow - Shadows are those that found no joy in life or god. They exist two dimensionally and are horribly wrathful beings who spite everyone and everything. They are humanoid shaped but are of the darkest black, and move very quickly. Shadows can be exceptionally malicious - carving out meticulous and deceitful plans who harm the living in any way possible.
Ghast - Ghasts are dead murders or those who took delight in unchecked violence. They appear as slightly dessicated dead that move with frightening speed and determination. Ghast will fly into a berserk rage in combat, leaping unto victims and ripping them apart with nails and teeth. Ghast have an innate cleverness but lack the comprehension for elaborate plans or motives. They are mostly driven to murder and violence with little thought to anything else.
Wraith - Wraiths are nearly insubstantial, and are the undead apparitions of those who killed themselves. They have given their bodies away through suicide and so linger in a mournful state that barely exists. Their presence brings forth despair and lonely emotions of all those in the area, and they consume anything joyous or vivacious around them leaving nothing left. Wraiths eventually drive the living near by insane, but are difficult to thwart in their near-invisibility state and lack of need to do conflict.
Mummy - Mummies are the remains of exploiters of their fellow people. This most often includes tyrannical rulers or others that were in positions of abusive power over others. They usually remain in the state which their body was placed. Maggots, vermin, insects or excrement form where ever they step - which represents the words they speak. Mummies seek power and exploitation and often control skeletons and zombies.
Vampire - Vampires are those which fed on the sorrow of others; opportunistic dealers of vice or sorrow. and deceivers who brutally harmed others for their own gain. these often include barrators, nobles, or authority figures which took advantage of others with no regard for their being. Vampires are very much the blood-sucking undead of lore, but also have dark hands which drip of sticky tar - this represents the sticky fingers and dark secrets which they fed upon. Vampires can occasionally hide their undead nature with ample clothing.
Ghost - Ghost are the worst of thieves or those who engaged in deceitful skulduggery. They have no substance and can not act upon anything. Ghosts loose their very identity, which occasionally gets lost onto the living. Ghosts, nearly invisible can wreck much havoc before being driven away if their personality is lost unto a living person.
Spectre- Spectres are falsifiers and frauds who have betrayed their core beliefs; either their liege, faith or those who trusted them. These undead are dessicated corpses burned black from cold, all except their head which remains perfectly preserved. Spectres deal in whispers, lies, and falsifying items. Spectres take joy in breaking the faith or trust in others, and relish in the chaotic aftermath. Spectres can occasionally hide their undead nature with ample clothing.
Lich - Liches are sorcerers, astrologers, and false prophets of malefic intent. Many have there have their heads twisted around on their bodies backward, so that they found it necessary to walk backward because they could not see ahead of them. Liches continue to decay, even in their undeath until they become dust. Some decrepit Magic-Users intentionally place themselves in this state to gain more power, for it is said to be the only true way to learn black magic.
Orcus - Orcus is a unique being, an undead blight. Others may exist like him, but they are unknown. Orcus, the lord of the dead, was once a mortal who personally affronted the great Deity in an unknown but apparently most dire manner. Because of his unknown crime, he was deemed to languish in a state of perpetual and horrific torment. Orcus appears as a crushed and mutilated body, like a piece of half-eaten food that is constantly being chewed up, some-what reconstituted and chewed up again. His pitiable wails and other unwholesome noises drive any sane being away from him. All animals flee from him. Any vulnerable dead who hear this noise, rise and then flee.