The Zombies in the book The Rising and City of the Dead are flesh eating corpses that are possessed by demons known as the "Squissm". Their leader is Ob.
The undead or rather demons are very intelligent and have good motor skills. Once a demon possesses a corpse the demon has all the memories as the host; they can plan, use weapons and drive cars, tanks and many other vehlicals. Near the end of the book City of the Dead the zombies use tanks and heavy weapons to kill everyone in a human strong hold.
They originate from a place called the Void. The Void is described by Ob as very cold and there numbers are "more than infinity".
How to become a zombie
A bite is NOT infectious. Zombies will eat part of you so you die and the demons possess the corpse. If you die (unless if a brain injury) you WILL become a zombie. However, you can die from a zombie bite because there are diseases carried by the zombies (Hepatitis, Bubonic Plague, MRSA and many others).
How to teriminate a zombie/demon
You destroy the brain. A gunshot to the head will do it or any blunt object.
Who becomes zombies
Humans and animals; it is confirmed that rats, dogs, cats, lions and many other creatures can become flesh eating zombies/demons. It is revealed in City of the Dead that once all animal life is exterminated then other demons will take over plants and bugs then once that is done then the world will be destroyed by fire.