When something dies, its soul moves through the Astral Plane, guided by the Raven Queen to the outer planes. Meanwhile, their body starts to decompose and its material becomes part of the ecosystem again. Except, neither of these things are guaranteed.
When the soul doesn't move on, it might be because it's trapped, refusing to die, or just not know how to let go. The body might be reanimated and serve as a tool for those walking among the living. Sometimes the dead soul is even tied to its corpse in a twisted way of cheating death.
In almost every part of the world, the undead are shunned. Most don't want to see the husks of their lost loved ones shambling around if nothing else. And several of the gods consider them abominations.