(Information taken directly from Wikipedia.)
The Chitauri first appeared in The Ultimates (vol. 1) #8 (Nov. 2002).
The race called Chitauri appeared as Ultimate Marvel's (Earth-1610's) counterpart to the Skrulls. They are a shapeshifting alien species who have attempted to conquer the Earth, most notably during World War II and again in the early 21st century.
The Chitauri claim to be part of "the immune system of the Universe", wiping out disorder and free will wherever they find it. They seem to prefer to act behind the scenes, mimicking and influencing the social and military methods of the species they are currently infiltrating. For example, they aided the Nazis in their attempt at world conquest by providing them with the technology to create a nuclear bomb carried by an intercontinental ballistic missile. However, this attempt was thwarted by the prototype super-soldier codenamed Captain America. Through the entire conflict of World War II, the Chitauri were being driven out of their operations in Africa and Europe, even from their main Japanese training camps in Hiroshima and Nagasaki. Following the end of the war, the Chitauri withdrew to make new plans.
The next attempt at conquest was more subtle (at first), involving long-term methods of manipulation such as will-inhibiting drugs in many nations' water supplies, influencing the media, and R.F.I.D. (radio-frequency identification) microchips to be implanted in schoolchildren, among other means. The Chitauri also infiltrated S.H.I.E.L.D., particularly the Psi-Division which could telepathically ferret out Chitauri agents. However, S.H.I.E.L.D. was able to detect some of the low-ranking "drone" staff of the aliens, disguised as common office workers, and wiped them out in an assault led by Black Widow and Hawkeye. The Chitauri planted false information through the compromised Psi-Division that led S.H.I.E.L.D. and the Ultimates into a trap on a small Micronesian island; due to the combined efforts of Iron Man and Thor, the Ultimates, Nick Fury and a handful of S.H.I.E.L.D. soldiers survived, but thousands of S.H.I.E.L.D. soldiers were killed and dozens of Helicarriers were destroyed.
The Chitauri then counterattacked by infiltrating the Triskelion and capturing The Wasp. The Chitauri leader (known only by his former Nazi identity as Herr Kleiser) took the Wasp to a hidden Chitauri base in Arizona. However, their location was given away when a fleet of damaged Chitauri starships suddenly entered Earth's atmosphere to hover above the formerly secret base, claiming that they were fleeing major defeats across the galaxy by their intergalactic enemies and that the remainder of their forces are forced to the "backwaters" of the Milky Way Galaxy (i.e. Earth's solar system). Disregarding Herr Kleiser's long efforts, the Chitauri ordered him to destroy Earth and its solar system with a doomsday bomb as part of a scorched-earth policy and retreat to the "lower fourth-dimension". The Ultimates and all available S.H.I.E.L.D. and military forces immediately converged on the alien fleet.
Although Iron Man and Thor were able to damage the ships (as well as disposing of the Chitauri bomb by teleporting it to the wastes of Nastrond, where its detonation caused only a small ripple in space-time), it was the Hulk who downed the majority of the fleet. At the same time, Captain America battled his old enemy Kleiser, but was unable to defeat him alone. At Captain America's urging/mocking of Kleiser touching Betty Ross (Hulk's crush), the Hulk was able to beat, dismember, and devour Kleiser. It is presently presumed that the combination of the Chitauri's series of intergalactic defeats and the destruction of the Chitauri fleet on Earth spelled the complete and total defeat of the alien race.
Millar's conception of the Chitauri was inspired by the British conspiracy theorist David Icke, who believes that the world is secretly run by an elite called the Illuminati who are really shape-shifting reptilian humanoids. During the Chitauri-Kree War, an entire Chitauri (whose true forms resemble their Avengers counterparts) armada was fighting their enemies when they encountered Gah Lak Tus. When their ships were being heavily damaged by the entity, a rift in the fabric of reality brought a being from another universe, which called itself Galactus. Gah Lak Tus merged with the visitor, and created an even more powerful Galactus with need to feed.
The Chitauri have appeared in the animated movies Ultimate Avengers and Ultimate Avengers 2. They have also appeared in the live-action film Marvel's: The Avengers.