Headcrab Zombies are unconventional zombies first appearing in the 1998 first person shooter, Half-Life and its spin-offs as well as sequels. They also appeared in the fan remake of the original game using Half-Life 2's source engine, Black Mesa.
On an interdimension drop off point known as Xen, sometime after the Nihilanth had enslaved and took command of the Vortigaunts, the Nihilanth had created the Gonarch, a giant creature, in order to to produce miniature 'headcrabs'. The Nihilanth had noted scientists from the Planet Earth had constantly arrived at the point, and after they took in the crystal sample "GG-3883", the Nihilanth readied it's forces for a full on invasion of Earth, as the sample was noted as the 'purest, but most unstable'.
On the month of May, the 16th, staff at the Black Mesa Research Facility had unintentionally opened a rift between Earth and Xen, causing a Resonance Cascade. Multiple races, both militant and wild, poured only into Black Mesa, slaughtering and eating scientists, and civilians alike. Headcrabs were among the many races that invaded Earth.
Upon entering Earth, Headcrabs roamed around Black Mesa, that is, until they encounter a human. They would then leap onto the human's head and crack open the skull, and gain access to the human's nerve system. Keep in mind that the human would still be alive at this point, and in excruciating pain. The conversion process is debatable, but most likely between five to thirty minutes. Gordon Freeman first encountered these creatures a few minutes after leaving Test Lab C/33a.
Headcrab Zombies possess the body of the average human, male or female, with a Headcrab latched to the host's head. Due to the Headcrab's slow adaptation process possessing a human, the host tends to stumble towards their pray slowly, like a typical zombie.
Key characteristics are their incredibly long and sharp claws which cause severe lacerations, incredible strength, as seen when they are able to lift Black Mesa security officers up with ease, and a large chest cavity that is used for feeding.
Despite the Headcrab having complete control over the host's body, the host still remains concious and alive until the Headcrab is detatched or the host is killed. In Half-Life, Headcrab Zombies would make alien like groans and grunts, followed by a slight chuckle. In Half-Life 2, however, the host would groan in extreme pain and when set alight, cry. They still retained some of their alien like sound. If one were to reverse some of the standard Headcrab Zombie's audio, it would reveal someone shouting "Help, god! Help me!" or "It burns!".
After one day, a Headcrab Zombie would evolve and transform into a Gonome. Gonome's were much taller than the average human, standing at atleast six or seven feet tall. They were much faster than the regular zombie, and their chest cavity had mutated across their entire torso with green tumors appearing on the host's body. They could also reach inside of the cavity and throw a poison like substance at their prey.
Fast Zombies and Poison Zombies differ greatly from this. A Fast Zombie has their entire skin peeled off completely, with only their muscles and skeleton remaining. Poison Zombies are much larger and stronger than both Zombies, and are hunched over, and the slowest.