These are the second stage of the Xenomorph life-cycle before the Chestburster. The eggs, upon sensing nearby prey, will hatch and release a Facehugger that will immediately wrap itself around the target's head and neck, using its tail. (Attempting to forcibly remove the Facehugger has shown to be fatal to its host.) Once affixed, it will lay an egg down the victim's throat, after which the Facehugger will die and release its victim afterwards. Although the victim will seem fine at first, it's only a matter of time until the embryo hatches, and bursts through the victim's ribcage. The resulting Xenomorph will have different physical characteristics, depending on what type of creature is their host. A human-hatched Xenomorph for instance has shown to be very different from a Predator-hatched Xenomorph. Some believe that Facehuggers would be useless against targets smaller or larger than their grasp is, but Facehuggers would then more than likely employ a different method in which to impregnate the victim.
Facehuggers typically overwhelm their victims with sheer numbers, but can also work in conjunction with other Xenomorphs - at times, Xenomorphs have even been shown to carry Facehuggers with them.
There are also situations where a human is captured and brought back to the hatchery, where they are cocooned and forced to await the hatching of a nearby egg.
Powers & Abilities
Despite their limited lifespan relative to their primary function of impregnating other species, Facehuggers have a host of capabilities. According to Science Officer Ash in the first Alien film, a Facehugger is made up of a combination of protein polysaccharides and polarized silicon, making it practically invulnerable and "one tough little son of a bitch!" Even if sufficiently injured, Facehuggers will bleed the same acidic fluid that Xenomorphs are known for, and can heal relatively quickly from lesser injuries. They are highly agile creatures, able to leap great distances with their tails to get to a target's face, and can crawl around with frightening stealth and speed. Once attached to a victim, Facehuggers are virtually impossible to remove without killing the person, as they have an incredible grip strength and even seem to partially bond to a person's skin in order to prevent being removed. They can determine what their host breathes and filter this from the surrounding air and into the host's mouth to keep them alive, and are able to incapacitate them while attached to their face - how this is done is uncertain, but the speed with which Facehuggers can incapacitate a victim seems to suggest the use of a toxin of some kind. Facehuggers' tails are also very strong and can easily strangle an uncooperative host, or may threaten to do so if attempts are made to remove it. It is unknown whether Facehuggers need to breathe in any way, but it seems unlikely as they easily recover after being submerged in preservative fluids for lengthy periods of time.
Overall, Facehuggers seem to carry many of the innate survival capabilities of full-grown Xenomorphs, and operate on much the same form of instinct rather than complex consciousness.