Wights are much like zombies in general. They were people who died and then whose souls were bound to their dead bodies. However, unlike normal zombies they were buried in mass graves. Usually this is the site of a mass grave after a battle during war. They are old enough so that they have lost much of the flesh on their bodies, appearing at least partially skeletal, with burning red eyes.
Some believe another way to create Wights are for a human to die while in the Fey Otherworld realms.