Undead is a rather generic term referring to creatures and people which are dead, but still mobile. The undead tend to have four major classifications. The first is whether they are physical or spiritual. Basically, anything with a body is physical (such as a zombie), while a ghost would be a spiritual undead. The second classification is whether they are mindless or intelligent. Intelligent undead are just as cunning as any living creature, and have control of their actions. Mindless undead either lie under the control of another, or instinctively seek to extinguish the living.
There are numerous origins for the undead. These include:
1. Sometimes they are created with a purpose, such as when an necromancer raises a cemetery to create an unliving army.
2. Other undead are created when someone suffers an untimely death, but choose not to move on, such as a ghost seeking to punish their murderer.
3. Sometimes the undead arise spontaneously, especially if there is a large number of deaths nearby (wars are good at creating the unloving).
4. Finally, undead may arise under special circumstances, such as a skeleton rising to attack a tomb-robber.