The dead are a frightening foe indeed. Unrelenting, unfazed by even the loss of limbs. Hard to fight an enemy when they continue to get back up. Harder still when they're you're own friends and family.
Zombies are among the most basic undead. They lack any trace of the soul that once inhabited their body, serving as puppets under the control of the necromancer or other power that raised them. The walking dead lack motor skills, unable to speak or operate tools. Only the most basic of commands work on them too, such as "move," "stop," and ofcourse, "attack."
Zombies are most often used by necromancers. They come in a number of different forms based on the many different subtle types of necromancy used to summon them. The undead nation of Necropolis employs zombies as foot soldiers. Heretics create their own type of zombie soldiers, and the Brood's most basic infection functions much the same as a regular zombie.
Of special note are the Wizened of Mesonoxia, who see the living dead not as mere soldiers in an army but as work horses. The Wizened's mastery of death has allowed them to imbue new life into the dead, overcoming the basic intelligence of the zombies and allowing them to serve many other labor roles.