Featured in Alien, Aliens, Alien 3, Alien Resurrection, Alien Versus Predator, and the upcoming Alien Versus Predator: Requiem, a Xenomorph is a particularily nasty creature.
First, a queen lays an egg, which opens and releases a Facehugger, that proceeds to latch on to someones face, and implant an embryo into their throat. The embryo grows until it no longer fits, and then eats and smashes through the chest of the victim, most likely, killing it. The newborn Chestburster must feed to grow into an Adult.
"A Xenomorph may be involved," ~Lieutenant Gorman
"It's a but hunt," ~Corporal Hicks.