The Undead as a class, encompass a variety of different things. In general, though, the undead are anything that was once dead, but now has some semblance of life. Living things resurrected by holy magic or life magic are excluded, and anything created with necromancy is usually included.
Nearly all undead, like most cursed individuals, have an unnatural aura, which animals (especially horses) can sense and are terrified of.
A near-infinite variety of undead can exist, limited only by the talent and creativity of a necromancer. The most common forms include the following:
Ghosts Ghosts are the metaphysical remains of deceased persons who have not yet wandered into the Shadow Realm usually for natural reasons. Most commonly, ghosts will remain if the manner of death was violent, or if the ghost has some connection to something in the world.
Lichs Lichs (or liches) are the most powerful of undead creatures. They are a special case, rarer, as undead go, but still some of the most feared and terrifying of any monsters. Soulless and immortal, they are free-willed and evil, and even if there were mortal men stupid enough to attempt to control one, it would be impossible to summon the power.
Skeletons Generally, the skeleton is an internal array of bones (or external case of chitin, in the case of exoskeletal creatures) that serves as a support structure for most animals. In Vaxian usage, however, it usually refers to animated skeletons, which are undead creatures like Zombies but whose flesh has all decomposed away and who are usually given weapons. They are animated by necromancers, who control them like puppets.
Vampires Vampires in Vaxia are mostly undead, individuals who have been cursed and brought back to a form of half-life by another vampire. There are many different clans of vampire, with differing abilities. Vampyres are just the same, the only real difference being that they can't spell correctly.
Vaxia/Races/Undead/Wights Wights are undead creatures that are similar in construction to Vaxia/Races/Undead/Zombies and Vaxia/Races/Undead/Skeletons, in that they are raised from dead bodies by necromancy. However, they somehow have slightly more free will than the others, and are often given armor.
Wraiths : Wraiths and Shades
Wraiths and shades are barely-corporeal undead creatures which are created by necromancy. A necromancer fishes a recently-deceased soul out of the Shadow Realm and gives it form. The resulting creature is fairly independent, but not sapient, and can best be described as rage given form. If the necromancer fails to keep tight control over his wraiths, they may turn on and kill him. They are something like thick black clouds or thin black sheets, and are often, but not always, given metal claws.
Zombies are undead creatures with no mind of their own. A necromancer raises a fresh corpse from the dead, and operates it as a puppet.