Undead are entities that are dead but still walk the living world of Glorantha. They are beings of Chaos and, during the Chaos Age, Wakboth and his lieutenants muster an army of undead to destroy all of reality. Examples of undead beings are vampires, skeletons, and zombies. Nontraya is the god of undeath while Vivamort is the god of vampires specifically.
Death gods, like Humakt and Jajagappa, are enemies of the undead. It is the mission of their champions, like Kerenna or Korestene, to destroy all undead beings they can find.
In the confused time of the Chaos Age, some dead may simply not realize the state they are in and well-meaningfully bother the living. Such situations may better call for gods such as Yinkin.
Ghosts share many characteristics of the undead—and both manifest more easily when the line between the worlds of living and dead blur—but ghosts are not necessarily a product of Chaos.