Xenomorph

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A deadly monster from the Alien movie, comic and novel series, Xenomorphs are fast and powerful creatures which live in a hive like society.

These aliens are roughly humanoid, but have an insect like appearance. They have many distinguishing features including:

An elongated, cylindrical head with no eyes
A black exoskeleton
A prehensile tail with a sharp stinger at the end
Acidic blood
A secondary jaw which can extend from the mouth
Sharp claws
Ability to cacoon victims and alter structures to make a den
Speed, agility and the ability to stick to surfaces
Does not give off any heat

Life Cycle

Egg
The egg is the Xenomorphs first stage, and it remains dormant within until the approach of potential prey. It is unknown how long exactly an egg can remain functional, but it is probably similar to a plant seed and well beyond the life-span of humans.

Facehugger
After hatching from the egg, the creature has the rough appearance of a large yellow spider. In this form, the aliens purpose is to find an appropriate host in which to lay an embryo.

When an appropriate host has been found the facehugger, as its name implies, attempts to grab onto its victims head. Once attached it quickly renders the host into a comatose like state and seems to glue its legs in a locked position. The victim is fed oxygen directly from the facehugger and is kept alive by it until it is released. Once impregnated, the facehugger drops off and soon dies.

Chestburster
After the host is impregnated by the facehugger, a small parasitic alien inside begins to develop. Once it is ready it proceeds to chew through the host until it emerges from the upper torso. This is extremely painful and fatal to the host. At this point the alien resembles an insect larvae and is only about two feet long. It hides as soon as possible as it is quite vulnerable at this stage. A chestburster only needs a few hours to grow to its full size.

Adult / Drone
Roughly eight feet tall, this is the alien once it reaches its full state of maturity. Drones can differ depending on the host that they spawned from.

Queen
The mother of an alien colony, the queen much larger and more intelligent then standard drones. She is first seen attached to a large eggsac, but she is able to break free of it when agitated or endangered. Asides from the difference in size, the queen is also covered with thicker armor, has a much larger cranium, and has two extra sets of arms.

Praetorian
A royal guard to the queen. Praetorian's are larger then drones, but still only about 1/2 the size of the queen herself. Praetorian's are not in the alien movies and many fans do not consider them canon.

Variations

The Alien has been portrayed in many noteably different ways throughout the films. Much of this is due to the technology and techniques used to bring it to life.

Body Shape
In the movie Alien, it was portrayed as a rather bulky creature by an actor in a large costume. By the time of though, they were depicted as much faster, more slender creatures created by computer graphics.

Head Shape
Another source of debate is that the aliens are portrayed with both smooth and ribbed heads in different films. There is much debate amongst fans as to what the difference represents (drone vs soldier seems to be the most common thought), if there is one at all.

Fingers
The original Alien had six fingers, whereas the ones in the sequel had three.