The Headcrab, also referred to colloquially as "head humper" by Barney Calhoun and "parasitic" by the Combine Overwatch, is the most numerous and arguably iconic alien of the Half-Life series.
Headcrabs are fairly small creatures, consisting of a rounded body with four legs for movement and a pair of larger frontal claws for attacking prey or as additional support when standing still. Headcrabs appear to have no external sensory organs, so it is assumed they move about by touch and vibration.
Headcrabs are a parasitic species, so far only known to affect humans and transhumans, attaching to the host organism via the head, the headcrab then proceeds to take over all of the host's voluntary systems, turning them into a zombie, it is likely the victim is killed in the process. It is unknown how long "zombification" takes, although the host appears to retain atleast some conscious mind some period of time after the headcrab attaches, as seen in Uplink where a scientist with a headcrab attached to his head (before zombification has taken place) remarks about how dark it is. However, shortly after the bombardment of Station 6 through the use of Headcrab Shells by the Combine, Gordon Freeman encounters Standard Zombies, showing that the zombification may take from several seconds to minutes.
The Headcrab includes several subspecies. The first to be introduced were the Standard Headcrab(in adult and infant version) and the Gonarch in Half-Life. Half-Life 2 updated the Standard Headcrab (though only their adult version is present) and introduced two new subspecies, the Fast Headcrab and the Poison Headcrab, along with new Zombies.
In Half-Life 2, the Earth-occupying Combine alien faction is seen utilizing Headcrabs as a form of biological weaponry against the Resistance. "Headcrab Shells", as they are known, are mortar shells containing several Headcrabs. After impact, their payload of Headcrabs is released into the open, free to infest or kill nearby victims. If used in large numbers, these shells are highly effective in neutralizing a large concentration of Resistance groups. The effects of large-scale Headcrab "shelling" are seen during Gordon's visit to the devastated town of Ravenholm.