The Chitauri are extraterrestrial shapeshifters and enemies of the Avengers in the Marvel universe.  They usually

A Chitauri in The Avengers (2012)

 take the form of humans to trick other people.  They claim to be "part of the immune system of the universe" wiping out disorder and free will whenever they find it.   They first appeared in the Ultimate Marvel universe as counterparts to the Skrull, another race of shapeshifting aliens.  They are also enemies in movies such as the Ultimate Avengers animated movie, Ultimate Avengers 2, and the 2012 live action Avengers, where Loki made an alliance with them to get revenge against Thor.  However, in this adaption of the Avengers Chitauri are not shapeshifters and are shown to be weak footsoldiers.