Because there is so much info on the adult alien, Physical Attributes will be divided further:

An adult alien stands between 190 and 220 cms tall, (about seven or eight feet for you imperialists) and is vaugely humanoid. It has a segmented exoskeleton and 5 spines of their back for protection, as well as acid blood- some alien even have the capability to spit acid. They are usually black, but often with traces of blue, green or brown. They have normal jointed limbs, but in some cases they have been known to have triple-jointed legs. Most aliens have five fingers and one thumb, but the first two pairs of fingers are 'joined' together. They have a long, skeletal tail with a barbed end that is often used as a weapon.

The long, tooth-filled head is the most prominent feature of an adult alien, their eyeless gaze responsible for terror in many sleeps. The alien has two sets of mouths, the second inside the mouth and also acts as a tongue. They can extend this 'double jaw' to about one foot with incredible force- the primary use is to ram through the enemy's skull. Without eyes, it has been suggested that the aliens move about and hunt using very advanced forms of sonar or scent, what exactly remains a mystery- but one thing is for certain, their lack of eyes hinders them in no way. One of the most obvious changes in appearance on an alien head is some have a smooth, (right) translucent head some some have a ribbed head, (above and below). The theory behind this is:
When aliens are only young they have a the smooth protective 'case' over their developing 'ribbing'. After a matter of days, when the ribbing is fully developed, the alien sheds this case like it's skin, revealing the ribbed head. The ribbed head is a better receptor of telepathic signals from the queen.

The aliens are linked to the queen via telepathic or telekinetic wavelength. Driven by the queen, and with all of the warriors combining with the queen's intellect, they create a 'hive-mind' style matrix- meaning the whole hive thinks as one. The warriors themselves aren't that smart- only the intelligence equivalent of a dog. But their sheer will to survive and cunning is far greater than anything seen before. They go on suicide missions purely to ensure the survival of an egg, chestburster or, most importantly, their queen. 

Aliens behave in very different ways in different environments. In a hive situation, they will 'coocoon' themselves to the hive walls and wait in ambush for their prey, unless their is a source of food or hosts nearby. In that case, they will work out when their target in most unprepared, and attack according to the enemy's strength, from scout runs to swarms.
Scout runs are when single or small groups of aliens are sent out to attack small targets, whereas Swarming is sending a huge force of warriors in waves towards the whole enemy, overwhelming them with sheer numbers.

In a non-hive environment, as seen in Alien and Alien3, single warriors or groups will always work stealthily and sliently pick off targets one by one, either killing them for food or coocooning them to walls to be impregnated.

Single aliens in a non-queen envirnoment can lay eggs, albeit very slowly compared to a queen. Aliens without a queen can also metamorph into a queen over a matter of time.

Because aliens will always  use some of their host's DNA, they always bear resemblance to the host, causing strange alternate forms of alien such as the alien dog and predalien.
There is some speculation about and alien 'drone', but the queen is born fertile to ensure that a soliatary queen can create a whole hive without any other alien. A drone alien is simply an alien that guards eggs and / or the queen, with no biological differences to a warrior.

In Alien Ressurection, there was a 1/2 alien/human hybrid known as the newborn, but that's covered later. Bluurrk!