The Chitauri are a sentient species of cybernetically enhanced beings operating under a hive mind intelligence. Subservient to the overlord Thanos, they were most notable for being the first major threat to the Earth that required the formation of the Avengers when they attempted to commence a planetary invasion as part of an alliance between Thanos and Loki.
The Chitauri were Thanos' infantry during his attack against Zen-Whoberi, where he massacred a half of the Zehoberei species, in order to prevent overpopulation from condemning the planet.
When the exiled Loki allied himself with the warlord Thanos, the Chitauri were granted to him as his personal army to conquer Earth and gain the Tesseract.
Once having gained the Tesseract from S.H.I.E.L.D., Loki was able to open a portal to Earth, with the Chitauri fleet standing by, thanks to the help of a brainwashed Erik Selvig. The Chitauri immediately started to wreak havoc on New York City, destroying buildings and gunning down civilians in the streets.
Luckily, the Avengers arrived, having fought Loki repeatedly in an attempt to hinder his efforts at summoning the Chitauri. Soon after their arrival, a major battle ensued between them and the Chitauri.