The alien lifecycle starts, like so many other things, with an egg. The leathery xenomorph eggs are produced by a queen, and holds within them a creature known as a "facehugger". The egg also seems to function as a simple organism in itself as it detects when a potential host is near, in which case it wakes up the facehugger and opens itself up. The term "wakes up" is used because it seems like the facehugger is in some kind of hibernation state inside the egg, and it is capable of staying alive for extreme periods of time in this way. In the first movie Alien, eggs were found in a ship that had crashed such a long time before the Nostromo crew found it, that its pilot had become fossilized, and they were still fully ready to release a facehugger within a minute of detecting a host.

So what is the facehugger then? Well, they get their nickname from the way they jump up out of their egg and attach themselves to ("hug") the face of the poor host organism who got too close. Once the facehugger has attached itself by gripping the victims head with its long legs and wrapping its tail around its neck in a choke hold, it renders the host unconsious while still keeping it alive by actually breathing air into its lungs. It then slides a tube down the hosts throat and implants an embryo that feeds off of the host and starts growing at a rapid rate. Once this is completed, there is no further meaning to the facehuggers life, and it detaches itself and crawls off to die. As the facehugger has - besides a mean grip and ability to choke its victim - acid for blood, it is practically impossible to remove unless there is access to an advanced medical facility.

Once the embryo - in a matter of only hours or days - has grown into what is known as a "chestburster", it does like the name suggests and simply bursts its way straight out of the hosts chest. This is for obvious reasons a most painful procedure on the poor hosts part, one that results in certain death. Once the chestburster has broken free from its host, its on its own and will roam around looking for food so it can grow into the 7-8 foot tall adult alien it eventually becomes at the end of its lifecycle.

As far as fully grown adult alien drones go, they seem to all have the same basic appearance in that they have a long pointy tail, an elongated head and so on, but they also seem to change just slightly depending on what kind of host they sprung forth from. The general understanding of this is that the alien embryo somehow borrows certain DNA sequences from its host and incorporates these traits into its own genes. Maybe this is to make it more adapted for a life among creatures similar to its host, but this is mostly speculation.

There is also the possibility for the embryo to develop into a queen, which is considerably larger in size than the normal "drone aliens", and also capable of laying eggs. The laying of eggs usually occurs via a huge eggsack, but there is no definite answer as to whether this is actually needed for the egg production or not.