THE SPACE JOCKEY/PILOT
The Space Jockeys are an ancient race of spacefarers speculated to have created the xenomorphs, possibly as weapons for a civil war.
The desiccated corpse of a Jockey was discovered in the first Alien movie, when the commercial starship Nostromo set down on the uncharted moon LV-426 in response to a signal interpreted as a distress call. The crew found a wrecked derelict spacecraft with a dead life-form inside, apparently its pilot. The deceased creature's chest cavity was ruptured from the inside, foreshadowing the events to come.
THE PREDATORS/YAUTJA
The Predator or yautja are an extraterrestrial species characterized by their trophy hunting of other dangerous species for sport, including humans.
Bad bloods are predators who have forsaken their code of honor, which includes rules against killing unarmed or pregnant prey. They are hunted by the other predators called arbitrators. Predators refer to aliens as the hard meat as Kainde Amedha and to humans as the soft meat as Pyode Amedha.
THE ALIENS/XENOMORPHS/BUGS/KAINDE AMEDHA/HARD MEAT
The alien, also called the xenomorph is a member of the parasitoid extraterrestrial species that is the primary antagonist of the Alien film series. The creature made its debut in the 1979 film Alien.
Unlike many recurring enemy extraterrestrial races in science fiction, the aliens are not part of an intelligent civilization. Rather, they are predatory creatures with no higher goals than the propagation of their species and the destruction of life that could pose a threat. Like wasps or termites, aliens are eusocial, with a single fertile queen and a caste of sterile warriors/drones.
COLONIAL MARINES (HUMANS)
The United States Colonial Marines (USCM) are a group of space marines first introduced in the movie Aliens. The USCM is the United States' "force-in-readiness," who are at all times ready to operate in environments far from home soil. By the year 2179, the force has roughly 165,000 marines divided into four divisions, four aerospace wings, and numerous support groups. There is also a reserve division and aerospace wing with additional manpower of approximately 50,000 marines. The USCM, along with the U.S. Army, are part of the United Americas Allied Command. The primary duty of the USCM is to maintain the security of the United Americas signatories and colonies, and serves as the vanguard of any major operation. The USCM have fought on more than two dozen worlds, including Tientsin (8 Eta Bootis A III), Helene 215, Linna 349 and the colony world of LV-426, where the Aliens were first encountered. The Colonial Marines are likely best known for their use of the M41A pulse rifle, a futuristic assault rifle with an underslung grenade launcher, and for the M56 smart gun, a large machine gun which is actually worn by the user using a sophisticated harness mechanism. Also featured in the movie Aliens are the M240 flamethrower, the M4A3 pistol and the UA-571C Sentry Guns. The Colonial Marines also employ a variety of land and aerospace vehicles, including the M577 A.P.C., the UD4L Cheyenne Drop Ship and the Conestoga-class assault transport ship Sulaco, all of which are featured in the film. A variety of other tanks, artillery and anti-aircraft vehicles are also mentioned in the Colonial Marines Technical Manual. One of the Marines, Corporal Hicks, is seen carrying a personal weapon of his, an "old fashioned" military pump-action shotgun "for close encounters." Likewise Vasquez is later seen in the movie using a S&W Model 59. It is also implied by the mess-hall and cargo-bay scene in Aliens that the USCM are also highly trained in knife fighting (and are equipped accordingly) as well as close quarters combat, making every Colonial Marine a formidable warrior even without the use of firearms.
Sources: Wikipedia