Human zombies basically are just like minor faction human resistance fighters, with all the same behaviors and such.
AI zombies are less intelligent in general -- like the human zombies, they just run around in FRD mode, and with that same basic behavior. I don't think they get orders from the central AI, if I recall.
The big thing on zombies on both ends is that they are also un-reclaimable. So when the AI spits out a bunch of zombies, you can't just convert them all.
EDIT: Ninja'd.