In the Half-Life video games, zombies are human beings that have been possessed by headcrabs, parasitic alien animals from the world of Xen. Ever since the Resonance Cascade Event at Black Mesa, Dr. Gordon Freeman as been forced to battle against these horrific creatures many times.
When a headcrab latches on to the head of a suitable host, it uses its incredibly sharp beak to penetrate the victim's skull and begin assimilating the subject. The exact biological process of this assimilation is currently unknown, but the end result is the victim's body being horribly mutated by the invading headcrab cells and the subject's motor functions are hijacked by the parasite.
Standard zombies appear as disfigured shambling creatures covered in blood. The mutation leaves the subjects' fingers as spindly claws for ripping prey to pieces and the torso appears to have split open, revealing the zombie's entrails. Supposedly, this torso wound actually serves as the zombie's mouth since the human mouth is covered by the headcrab latched on to the head.
Zombies seem to be devoid of all sense of self-preservation and will relentlessly attack their prey regardless of danger. Standard zombies cannot move very fast and can only limp towards their prey, or crawl with their arms should they lose the use of their legs.
In Half-Life 2, zombies can be heard groaning in a dinstinct painstaking manner as if expressing pain and anguish and calling out for help. This could indicate that the headcrab host is still alive and aware to a degree, but given the mutilated state of the body, most notably the ripped-open torso, this is debatable. Further evidence that zombies may still feel pain is the fact that they emit muffled screams when set on fire.
Zombies can be killed almost as easily as normal humans can, but they do not seem to register injuries such as gunshots (which is confusing given that they scream if set on fire). If a zombie's body is too badly damaged, the headcrab will detach itself from its host body and seek out another host, usually going for whoever destroyed its last host body. However, the zombie and the headcrab controlling it will both die if one fires at the headcrab itself before it discards its host.