Zombies are reanimated corpses rid of souls. They have no will and are usually subject to a necromancer.
Reanimating a corpse requires some effort and knowledge. Necromancers often study and perfect these processes.
The most common way to reanimate a corpse consists of using 2 powders. First, one consisting of dried pufferfish poison (it may also be used to kill a victim whose corpse will be subjected to reanimation) then one extracted from datura seeds. Then the necromancer will use their Authority, in any language, to reanimate the corpse.
As to better preserve the body, which now has no immune system or active stem tissues, necromancers employ a number of techniques. The zombie's fluids may be removed and largely replaced by formol, and they may be covered in bandages (that is, they will be mummified).
Zombies are weaker than actual living beings, since their muscles are atrophied and often rotten. They are also more fragile for the same reasons, but applied to all tissues.
Due to lacking a soul, zombies are unnable to make decisions, being completelly subject to Verinomination. By default, they will do anything they are commanded to, but a necromancer can also command them to listen only to a set of people. They may disrespect orders if they are given a command in a language closer to the First True Tongue.
Zombies can be revitalized. That is, they may get a new soul and essentially "give birth" to a new being within their bodies.
A use of zombies is in ressurection. If one is able to reunite a body and a soul, under the right conditions, they may ressurect someone. This process is dependent on many factors and requires a difficult quest, demanding effort from a necromancer and from the deceased. The body must be in proper condition (as well as rid of the poisonous powders it had), the soul must also be conscious.
Famously, this happened to Bleris, the "blood goddess".