The undead, very simply, are the
dead once again risen to
life. Examples of this
phenomenon include
zombies,
vampires,
wraiths, and
lichs. The undead are universally considered to be
evil.
Most powerful members of the genre entered into the state of unlife willingly. Lichs, especially, are horribly evil magic users which delve into the forces of necromancy in order to prolong their lives and give them power. The weaker members of the undead genre are conversely, usually forced into the state by others. Zombies are generally assumed to be the bodies of the dead imbued with unholy force, to do the bidding of the raiser, or more rarely, the souls of the dead forced into their former bodies for the amusement of a necromancer. Vampires, usually, are the unwilling victims of a series of changing bites by a person who has already been transformed into a member of the walking dead.
Generally speaking, it's assumed that objects of holy signifigance do considerable damage to these nefarious creatures. Holy water, symbols of a religion presented strongly by a person of faith, priests, and paladins are the most oft used weapons in a fight against the undead.