A crab-like creature about the size of a small cat which is used in asexual reproduction
for some species.
The head crabs attach to the head of a host, and in some way use that host to form a fully-developed member of the species, which is generally much larger and scarier than the head crab.
Became famous under this name in Valve's game Half-Life. Half-Life's head crabs permanently attach to the host's head and turn it into a flesh-eating zombie. They prefer to dine on scientists, marines, and police officers. The original head crabs may be from the movie Alien.
These head crabs put the host into a coma after incubating it with an egg. The egg develops into a baby alien which then tears through the chest of its host, and perhaps eats it as well. There are several known instances, however, where the head crab has been removed, or detached on its own. The host continued life as normal until one day, quite suddenly, an alien ripped through his chest.
The name normally used for the little beasts from Alien is "facehugger".