There are several different phases to a Xenomorph's lifecycle, and different outcomes to what kind of adult Xenomorph will come out.

Egg

Eggs are first created inside of a Alien Queen, and then exit her body through her large abdomen. The Egg is a leathery sack approximately 2.5 feet tall. They are the very simple first step.

Facehugger

The Facehugger is what lays inside the Egg. After a certain amount of time, if a host were to come too close to a Egg, the Facehugger inside somehow senses it, the Egg opens up like a blossoming flower, and the Facehugger launches itself out and plants itself on the hosts face. It's long tail will wrap around the hosts neck, and it will grip the head with it's long eight legs-the Facehugger resembles a flat spider, in a sense. It has acidic blood-if it is cut, it will bleed the acid on the victim, and if it is tried to be pried off, it's tail tightens around the hosts neck, which eventually strangles him.

Once the Alien egg is planted into the hosts body, the Facehugger will flee, and die shortly afterwards. The host will wake up with no recollection to the event.

Chest Burster

As time passes, the egg that was planted inside of the host will begin to stir. Then, it will begin to claw it's way out of the hosts chest, through the centre of his ribcage. This obviously causes extreme pain, and as soon as the Chest Burster breaks through, the host dies. It will then flee at top speeds to grow and become more dangerous.

It's appearance changes in it's variations, but it is small, with a small cranium, pale skin and a long tail-almost like a snake.

Adult

There are many different kinds of Adult Xenomorphs-however, they share the same general appearance. After the Chest Burster has fled, it will wait and mature, shedding it's old skin-revealing the darker kind-and it's head will expand backwards into a crescent shape. It will grow large arms and legs, it's tail will be longer and much stronger (even to the point of being able to impale fully grown Predator's), and it's blood will be much more acidic. Xenomorphs have many distinguishing features-they do not show up on infra-red scanners, as for some reason they do not give out heat. Inside of their mouth is a second "mouth", in appearance-however, this is it's tongue, which has shown the strength to punch through a Predator's mask and through his head. They (apparently) do not have eyes-which would suggest that they "see" through the usage of a motion sensor type ability-which would explain how they can detect cloaked Predator's.

These Xenomorphs are the stuff of nightmares-they are vicious, cannot be reasoned with, and will never, ever give up.

Variations

There are several variations of adult Xenomorphs. They most well-known are the "Warrior" brood. These are the ones that appeared in both ALIEN and ALIENS-they have the long crescent head, dark skin, and deadly tongue. The next type was the "Stalker"-this is the Xenomorph that appeared in ALIEN3-it is suggested that Xenomorph's take the characteristics of the host from which they hatched. This would explain why the Xenomorph in the third movie crawled on all fours commonly, since it hatched from a dog. This brood is fast and agile, and crawls on all fours. Another brood seemed to appear in ALIEN-RESURRECTION. It had the appearance of a Warrior, but had darker skin, was able to easily swim through water, and was much more intelligent-to the point of laying traps for the marines hunting it. They are arguably the most dangerous shown thus far-it's intelligence was far greater than those of previous Xenomorphs.

Alien Queen

Alien Queen

The Queen is the centre of all Xenomorphs in the colony-it is what births them, and is the strongest, largest, and fastest. When attached to it's abdomen, it is stuck in place, and births the Eggs from which the Facehuggers launch. However, it can detach itself from it's own abdomen, and stand fully tall. It's appearance is like that of a adult Xenomorph, but has slight differences. It has a large, wide head, instead of the crescent, and it's main body is skinny. It's arms are long and powerful, as with it's legs. It is the size of a Tyrannosaurus Rex-it was a massive killing machine in the movie "Alien Vs. Predator".

The Queen's only job is to reproduce a.k.a make lots of little aliens in eggs. The queen comes with a huge "sack" in which the little bundles of alien joy are stored. when the queen has a mood swing the sack rips in a grand explosion of slime/liquidy stuff. She then will proceed to chase you down until you are in her belly or......yeah pretty much in her belly.

Warning: Aliens are known to kill people and if ever you injure one (shoot the hell out of it) beware the acid blood, cause seriously folks this stuff will melt you faster than butter on the sun! Also make sure to keep your distance and always watch your back and make sure finally, to pump that thing full of lead/plasma/stuff until it is a smoldering pile of alien stuff.

Connection To Predators

Xenomorphs and the Yautja have a long history with one another. The Yautja will, when in need of a new Queen, raid a Xenomorph colony and steal one. They will then use that Queen to make several Eggs, and then they will plant the Eggs in on a planet. Inevitably, a Xenomorph outbreak will occur, in which the Yautja will send their rookie Hunters to the planet to claim their first kills. After the Yautja return from their first hunt, another group of experienced Hunters will come and exterminate all trace of the Xenomorphs, returning the planet to it's natural balance.

However, occasionally a few Xenomorphs escape the slaughter. If that were to occur, the planet is lost to the creatures.

The Predalien is what happens when a Predator is face hugged. Predaliens are a genetic cross between the Xenomorph and the Predator. They often Retain the Dreadlocks, mandibles, and jumping ability of the Predator. They also feature the mixed tenacity of an Alien and Predator.