Egg
Eggs are initially created inside the Queen, who uses her ovipositor to mature and then lay the eggs. The egg itself has a leathery texture (as observed by Kane) and stands at about 2 and a half feet in height, with a width of about 1 and a half feet. Some eggs have been observed to have tendrils around the base, or "roots", which have been suggested to possibly take nutrients from the surroundings. This theory may be supported by the fact that these tendrils are not present when the egg is first deposited, as they grow later when the egg has "matured". The life span of an egg is not known.

Facehugger
Facehuggers are found inside the eggs. It is made up of an abdomen, legs (or fingers), and tail. It does not appear to have any sensory organs on the surface. When a host nears a closed egg, the egg will open and the facehugger will attack. It is able to use the tail to propel itself with quite some force. The tail will wrap around the victims neck, and the fingers clamp firmly around the head. At this point, a retractable tube will extend from the underside of the facehugger, into the hosts mouth and down the throat. This is used to implant an Alien embryo in the hosts chest.

The facehugger has been observed to supply the host with oxygen, using the "lungs" which hang from either side of its body. How the creature determines its hosts biological needs is not yet known. Once attached, it is impossible to forcibly remove the facehugger without killing the host ("It's not coming off without tearing his face off with it." - Dallas). It cannot be cut off due to having highly corrosive "blood" at high pressure just beneath its skin. If it is removed before embryo implantation, the host will go into a state of shock that ultimately results in death. As Parker said, "It's got a wonderful defence mechanism - you don't dare kill it".
Once the embryo is successfully implanted, the facehugger will drop off and die. The host will slowly wake up from the comatose state, with no recollection of the event, unaware of the changes in their body.

Facehuggers are also adept at survival outside of the egg, using the legs to crawl aournd, reacting with surprising speed and agility. They would appear to have some sort of intelligence, using stealth to enable them to catch bigger, mobile and armed hosts.
They also adapt themselves to their surroundings -"He's got an outer layer of protein poly-saccharides, has a funny habit of shedding his cells and replacing them with polarized silicon which gives him a longer resistance to adverse environmental conditions." - Ash. This process enables the Facehugger to survive in extremely harsh conditions ranging from unbreathable atmosphere to extreme cold to prolonged submersion in liquid.

To be updated...