The undead should first be distinguished from things that simply resemble the undead. If, for example, through an xbox-huge amount of sorcery, you were to design a spooky scary skeleton who through sorcerous flows can move like a human, it wouldn't be undead. It would just be a construct made out of a dead guy.
True undead are corpses motivated by dark energies that corrupt the "Flame" that used to be their life's spark. This is the same Flame used by pyromancers, now turned evil and dark. It naturally corrupts all life around it, and generally ruins everything that touches it, but it is also strong. Very strong.
The undead are enormously strong, difficult to kill, and their mere presence saps the strength of the living, making them even more dangerous. Worse still, those slain by the undead often return as the undead themselves.
Undead infestations are serious business. Regular people have almost no chance of dealing with these things on their own, it requires extreme discipline, proper equipment, and knowledge to destroy them safely, and failed attempts only strengthen them. Schwarzvalds produce the undead only near the end of their lifecycle, if they successfully slaughter enough people. Witches and certain pagan magicians can also infuse corpses with the dark flame, but the undead are difficult to control, and few attempt this except as an act of spite or an experiment.
There are rumors of powerful undead who can control others of their kind, but if such beings exist, they are mercifully rare.