The Chitauri are a shapeshifting race of Skrull-like aliens whom had been infiltrating the human race for centuries. Their leader used the name Herr Kleiser. Their first big outburst was during World War II, where they tried to destroy the humans by allying with and infiltrating the Nazis. During that time, they were stopped by Captain America. Nothing was heard from the aliens in quite some time from that moment on.
Years later, in present time and near the end of the second arc of Ultimates vol 1, it is revealed that the Chitauri had already infiltrated most countries as some of their highest ranking officers, and were still attempting to take control of the human race. Their plans changed, however, when the U.S. government team known as the Ultimates became aware of the alien invasion. Threatened by the Ultimates, the Chitauri attempted to destroy planet Earth. This last attempt was stopped by the Ultimates team with some aid from the Hulk. Most of the Chitauri where killed during this final battle. They have not been seen since.
The Chitauri thus far appear only to be a variant of the Skrulls, found in the Ultimate universe. However, references in the main Marvel continuity hint that the Chitauri, or the 616 counterparts to them, are not related to Skrulls, but a more focused, Jihad-based defector group that believes in pure violence and complete destruction of perceived enemies. It is because of this extreme anarchist behavior that most Skrulls refuse to acknowledge the existence of this newer group, seeing them as cult-like extremists.
The Chitauri are similar to their Skrull relatives in that they are adept at infiltration using enhanced shape-shifting abilities. They are also very strong -- stronger than a modern human, very fast and increasingly agile. However, unlike the technology-based shape shifting ability of the Skrulls, the Chitauri transform by ingesting the organs and body of their victims, "becoming" them mentally as well as physically.
The Avengers (2012)
In the film The Avengers, the Chitauri are an invader race under the control of the mad Asgaardian trickster God Loki. The Other, a mysterious hooded figure, authorizes the Chitauri to follow Loki's orders, in exchange for the trickster finding and delivering to them the Tesserect, a cosmic cube of untold energy and cosmic power. While it appears The Other is the one the Chitauri answer to, it is in fact the vicious galactic villain Thanos that commandeers the Chitauri. The Chitauri in the film are very different that what is seen in the comics, being six digit invaders rather than reptilian infiltrators. However, while they do not seem to have (or at least show) their ability to transform into humans, they are highly adept at technology, using flying assault vehicles and piloting monstrous bio-mechanical space whales which they use to transport massive numbers of troops, much like an armored tank or Trojan horse.
The Chitauri in the film, despite being bigger and more durable than a human, are relatively vulnerable due to their lack of full body armor, only holding on to weapons and wearing masks to communicate (and possibly breathe). When Iron Man managed to destroy the main Chitauri war ship, all of the Chitauri fighting in New York immediately died, showing a vital connection between army and machines. They also, unlike the reptilian looking beasts in comics, appear more to resemble skeleton or mummies, or even mutated Korbinites, due to their distinct horse-like face.
It is unrevealed if the entire Chitauri race was extinguished in the invasion of New York, or if it was just one advancing fleet that was wiped out at the end of the film.
Guardians of the Galaxy (2014)
In Guardians of the Galaxy, a Chitauri soldier can be seen as a prisoner in the Collector's museum. He is still alive, suggesting that the Chitauri did not completely perish after Iron Man destroyed their main ship.