Headcrabs are one of the most iconic creatures in the Half-Life video game series. These alien animals, while small, are extremely dangerous to anyone ill-equipped to fend them off. Originating from the alien world of Xen, these vicious parasites were teleported in vast numbers to Earth via portal storms that occurred following the Black Mesa Incident. They are referred to as "headcrabs" because they leap out and latch on to human heads, then proceed to take control of their victims' bodies.
A typical headcrab is small, about 1 foot in height and 2 feet long. The creature's body is composed of smooth skin with a sickly tan pigmentation. It walks on four segmented legs, though its hind legs are capable of supporting its whole body, allowing it to "tiptoe" as it raises its forelegs when it prepares to pounce at prey. Headcrabs appear to have no eyes (though in the first Half-Life game they apparently have six very tiny eyes) and their fronts end in a set of four mandibles with fangs that inject toxins into the body of a potential host. Whether these toxins merely paralyze a victim or contribute to the zombification process is unknown. The underbelly of a headcrab consists of a gaping mouth containing a small but sharp beak. When a headcrab latches on to a victim's head, this beak burrows into the skull and the creature initiates an as-yet unknown biological process through which it hijacks the victim's body, controlling its motor functions and altering its physiology, causing the victim's fingers to elongate into gangling claws and strengthening muscle tissue to withstand attacks.
The headcrab can appear harmless at a glance: it is small and slow-moving with its tiptoe-like gait. Although it can quickly leap long distances using its hind legs, tilting its body upwards to turn its mouth towards its target, such an assault can only inflict minor injuries with the claws, legs and teeth. However, the headcrab's main goal is not to kill, but to attach itself onto an appropriate host's head. Using its leaping ability, it can reach a host's head and attempt to burrow its beak through the skull. Once successful, the headcrab takes control of the host's nervous system and causes physical alterations to the host's body, such as causing its fingers to grow into claws, via an unknown biological process. The host then rips open its own torso, revealing the stomach and other organs, for feeding purposes. Such a headcrab-controlled host is called a "Zombie".
A headcrab will frequently lie in wait in dark corners or ventilation shafts for unsuspecting prey to walk by. In more open environments, the creatures are known to bury themselves in the ground to hide, then climb out to the surface and attack when a possible host draws near. Headcrabs appear to be fairly unintelligent creatures and will pursue their prey under very dangerous conditions, moving through dangerous areas or towards hostile prey, even as fellow headcrabs die in front of them.
Headcrabs have been observed to be prey for Bullsquids, Vortigaunts, and Barnacles. Isaac Kleiner's pet headcrab, Lamarr, seems to have a taste for watermelons, suggesting that headcrabs are actually omnivorous, or possibly scavengers when not looking for a host. Headcrabs have also been seen cooked and eaten by humans in Black Mesa East, which suggest that they are used as a common food source after the extinction of most of Earth's species. Citizens in Half-Life 2: Episode One comment that "They don't taste like crab."
It is also apparently possible to "tame" a headcrab to some extent, as demonstrated by Lamarr.
Fighting off headcrabs can be tricky business when they are encountered in large clusters and ammunition is sparse. Gordon Freeman was able to kill many a headcrab using only a crowbar, but he wore a Hazard Suit for protection. Wearing headgear seems like a sensible way of keeping you safe from crab possession, but headcrab beaks are capable of penetrating a number of resilient materials and have been shown taking control of Combine soldiers.
Since their conquest of Earth, the Combine have been breeding headcrabs to use as biological weapons against groups of humans that resist their rule. Using captured Gonarchs, they produce them within their Citadel facilities and load them into shells which they then launch into target areas. The shells open on impact and the headcrabs are released to start attacking and possessing the locals.