The Dead is a catch-all term used to describe anything that has spent so long in Death that it can no longer live naturally and must be sustained with magic or the life force which can be had by killing the living. The vagueness of the term means that it can be used to describe entities as varied as enslaved corpses, to undead Free magic sorcerers like Chlorr. These are often divided roughly by their level of power into the Lesser Dead and the Greater Dead.
Lesser Dead can either be the somewhat free willed souls of strong willed but restless human dead, or unlucky souls which have been kept from their eternal rest by the enslaving magic of a necromancer. When under the control of a necromancer most Lesser Dead are mainly divided into the categories Hand (which require a recent corpse to posess) and Shadow Hands (which are deadly beings of dark spirit matter). There are however several types of more specialized Lesser Dead, such as the parasitic Mordaunts, the multi bodied Gore Crows and the horrifyingly powerful Mordicants.
The Greater Dead on the other hand seem to mainly consist of the souls of dead necromancers, which have used their strong will and magical knowledge to gather strength and influence beyond death.