Zombies in Doom 3 refer to a number of different enemies. Including the regular Zombies (in a variety of outfits), Zombie Security (aka Z-Sec), the Flaming Zombies (Bernies), Fat Zombies, Bonies and the Sawyer.
The basic Doom 3 Zombie is a callback to the generic "Romero zombies" (slow moving ghouls who feed on human flesh). They were once UAC civilians (or marines off-duty), now possessed by evil spirits. A scripted event early in the game shows scientist Jonathan Ishii coming into contact with a transparent "ghost skull", which turns him into a Zombie. The demonic force that creates Zombies does not differentiate between the living and the dead - slain humans will be re-animated as Zombies, and do not seem to be any less powerful than the ones created through possession of a living host.
Zombies come in differently-colored clothes, unlike the original former humans. They generally come in a number of flavors, each with different heads (or even no head) and different damage skins.
Zombies primarily attack by closing in on the player to slash them. Although they do not rely on using firearms like the Z-Secs, few may use wrenches and flashlights as clubs to compensate for more damage.
They come in somewhat different varieties, with the Fat Zombie a bit harder to kill due to its higher hitpoints, and the Headless Zombie due to not having a head that can be used to put it down more easily.
Zombies are often seen hiding behind dark corners or in "monster closets" in order to surprise any unsuspecting victim in the vicinity. Some Zombies also play dead to catch their victims off guard.
On lower difficulties, players can solely rely on using the flashlight, as two hits can kill a Zombie. On higher difficulties, the player should swap for a Pistol as the Zombie's clawing attacks do more damage than an Imp's attacks. The flashlight can also alert any Zombies that play dead if the light shines on them. This tactic will save the hassle of being ambushed by one.
It is best to maintain a reasonable distance against Zombies, especially when fighting against them in hordes. As they are slow and entirely melee only, the player should never back down to a Zombie in most situations. The Machine Gun can efficiently take down a horde of them. If you find yourself cornered, the Shotgun can help turn the tides as one well placed shot at point blank range can kill multiple of them.