The Chestburster is the immature infant form of the Xenomorph which emerges forcibly from the chest of its host, killing it. The chestburster is similar in form to the fully grown alien but far smaller and pale in color.
Its egg is laid in the host's bronchial tubes by a Facehugger, where it hatches. It then burrows for the host's aorta (major artery in the heart), where it feeds on absorbed nutrients and white blood cells. Until it finishes feeding on the aorta, its life is dependent on the survival of its host, but after it finishes, it will not require the host to survive (as seen in AvP and Avp:R) where a normal chestburster emerged from a victim even after he had been shot in the head (and a Predator X-Ray shows it had already detached from the aorta by this time) and the Predalien hybrid emerged from a dead Predator, which had been alive long enough for the chestburster to finish feeding. After about an hour or two, it lets go, and begins swimming about in the host's body, causing mild discomfort. Then, it finds the sternum, weakens the structure with acidic saliva and starts chewing. This causes the victim great pain, shock, and blood loss. Then, true to its namesake, it pops out, and finds a nursery or chicken coop, or, less messy, a food locker, something with a large amount of helpless meat. Its carapace has yet to harden, and is a dull grey fleshy hide. Depending on the host, it will grow to become a Drone (human, black carapace, but red upon emerging (standard Xenomorph), Predalien (Yautja, yellowish-green partial hide), or Runner (any given quadruped, red hide). Runners have all limbs, tail and inner jaw fully formed when they emerge. Genetic alteration causes it to split into identical twins after escape from a human or quadruped (AVP Extinction).
An embryo implanted in a pregnant woman will feed directly on the baby, and will erupt from the stomach of the woman. (Avp:R)
Behind the scenes
The chestburster was designed by Alien special effects master Roger Dicken. Giger had produced a model of a chestburster that resembled a plucked chicken and was far too large to fit inside a ribcage. Much to Giger's chagrin, his model reduced the production team to fits of laughter on sight, Scott drafted a series of alternate designs for the chestburster based on the philosophy of working "back [from the adult] to the child" and ultimately produced "something phallic," so Dicken was given the task to design it. Giger stated the finished form first resembled a dinosaur but the arms and legs were removed.
The chestburster in Alien 3 was far more developed, possessing complete limbs. The Queen chestburster depicted in Alien 3 has all four arms.