The Facehugger is the second stage in a Xenomorph's life cycle. With the Alien Egg being the first stage.
It is a Facehugger that attacked, and implanted the embryo it was carrying into, Catherine Foster. Thus setting into motion the events that took place aboard the Sevastopol Station.
These creatures are quick and resemble spiders with long legs, and a long tail capable of wrapping around a Human's neck. It has a surprisingly strong grip, meant to help a Facehugger succeed in its task of implanting its embryo. After it has succeeded, it dies shortly after.
Unlike its later stages, the Facehugger is quite vulnerable to all types of attacks. But due to its size and speed, it would be better to use a short blast from the Flamethrower. Using a Stun Baton is a much quieter method of dealing with them, but missing can result in your immediate death. They can also be smashed by swinging the Maintenance Jack with immediate death if you miss. It will also attract the xenomorph if it is close by.