The Alien film series is a science fiction horror film franchise, focusing on Lieutenant Ellen Ripley (played by Sigourney Weaver) and her battle with an extraterrestrial lifeform, commonly referred as "the Alien". Produced by 20th Century Fox, the series started with the 1979 film Alien, which led to three movie sequels, as well as numerous books, comics and video game spinoffs.
Related to the franchise are the "Alien vs. Predator" films (Alien vs. Predator and Aliens vs. Predator: Requiem), based on the related franchise which combine the Aliens with the Predators from the Predator film series.
Source: Wikipedia, "Alien," available under the CC-BY-SA License.