The origin of the Alien species is thought to be as some sort of genetically-engineered weapon, or perhaps as a means of terra-forming planets. Perhaps the constructs of the enigmatic "Space Jockey" seen in the movie "Alien". The original authors of the movie, and it would seem Director Ridley Scott anticipated exploring this idea more in-depth in the now-scrapped prequel movie, which has been divested of that original concept and is now being pursued as "Prometheus".
Later depictions of the Aliens might have done away with this concept entirely, so it cannot be considered decidedly cannon.
The Xenomorph, or Alien, in the form most recognizable is the result of gestation in a human, though it seems that Aliens have somehow evolved or been biologically engineered to be parasitic to organisms of multiple kinds and of completely different genetic heritage - meaning alien species of many different planets who developed completely independent of one another.
The adult Alien worker seems to copy or assimilate key genetic information from the host organism to better thrive in that organism's native environment. This also involves the adaptation of body plans to a certain extent.
For instance, an Alien warrior birthed from a dog, as seen in Alien 3, ran on all fours and seemed capable of moving at speeds not normally seen in previous outings, similar to how dogs can outpace running humans. The Alien does retain traits specific to the Alien, which don't seem to change depending on the host species, Including:
A smaller set of jaws attached to a piston-like tongue capable of punching through body armor, and human skulls. They do not actually posses a tongue, only another smaller mouth and to date the only Alien capable of impregnating human's orally through use of its inner mouth has been the Predalien featured in AVP Requiem and it has only been able to do so with human females.
Shaped somewhat like a banana, sometimes covered in a smooth hood, and other times exposing curvilinear skull protrusions.
Tubular Dorsal Venting
Usually depicted throughout various incarnations of the Alien is a set of four tubular vents which could serve as the breathing apparatus.
The tails have been shown grasping prey and used as spears or flails.
Very potent and dangerous corrosive blood fills the internal cavities of the Alien. When injured, the blood flows freely, shown to be capable of burning through several floors of human structures and humans themselves, but it has been shown to be harmless to other Aliens.
They are depicted excreting copious amounts of what is usually thought of as drool, which doesn't seem to posses the same corrosive properties as their blood.
Although the exact composition of the exo-skeletal armor of the Alien is not known, it does appear to be somewhat like that of the arthropods of earth.
Aliens have proven to be adept wall climbers, whether that is a result of digging into surfaces with their claws, finding hand-holds, or through some adhesive effect such as those used by geckos or insects has not been demonstrated.
Aliens usually behave as a hive, with one female producing the next generation. The Queen is the focus of the hive and seems to have some degree of communication and control of the Warrior Aliens. Whether that be in body language, as seen in the nest scene of Aliens, through audible communications, as seemed indicated in Alien Vs. Predator or pheromones as seen in insects on earth.
Likely some combination of these, or other elements not explicitly stated.
In a hive setting, the Warrior Alien is meant to capture and incapacitate live hosts to bring back to the nests where they will be impregnated with more Alien larvae. The usual method is to overpower a human, carry them back to the nest, and affix them to a surface with a kind of organic resin to prevent their escape. Later an egg will be placed nearby which can deliver the larvae and lead to the birth of still more Aliens.
When no hive community is present, the Alien has been known to generally kill other organisms on sight.
Though they seem to behave on a similar level of intelligence as earthly apex predators, such as lions, Warrior Aliens have demonstrated problem solving and the manipulation of tools to accomplish goals.
In various depictions the Aliens seem to acknowledge that their blood is dangerous and use it to ward off enemies, or in one instance they seemed to conspire together to murder a captive alien. Sacrificing the one so that others could use the hole melted by its death as a means of escape. Later in that same movie it was shown that the captive Alien had deduced the workings of a system that was used to torment the Alien in previous scenes, whereby the Alien presses a red button to douse a human with liquid nitrogen.
Physical Strength & Speed
Aliens are generally depicted as being superior to humans in such a way that a human generally has no chance of fending off an Alien with their hands. Aliens are depicted hauling fully grown humans up through the ceiling as though the weight was more or less inconsequential and have also been shown climbing with great speed directly up walls while carrying full grown humans.
In the Alien vs. Predator films, they were shown to be able to contend with Predators on purely physical basis.
Generally, the Queen Alien, which is massive, approximately 4 meters tall, lays eggs that stand about 1 meter tall. They look like leathery brown/grey eggs with four petals on top which unfold revealing a spider-like stage of development which fastens itself to the face (facehugger) of a potential host, inserts some sort of over-positor and plants a larvae in the chest cavity of the host.
In earlier depictions the gestation of the larvae into the early Alien developmental stage commonly referred to as a "chestburster" takes somewhere around 3 days. Variations of that time period were imposed for some reason or another from the much longer gestation of the Alien Queen in Ripley's chest in Alien 3, to the comically brief gestation of the larvae in Alien vs. Predators movies which seemed to take somewhere around twenty minutes.
Once the chestburster is free of the host, by means of literally bursting through the chest, it goes through a rapid metamorphosis where it increases in size until it stands roughly 7 to 8 feet tall (in the case of a human host).
This new Alien will either behave as part of a hive, if such is established, or be generally as murderous as you can imagine if it is alone.
Do They Have Eyes?
Aliens, despite popular belief, actually do have eyes which are located behind its exo-skeleton in the front of the head with its outer shell acting like a one-way mirror.