Grey aliens, also referred to as "Greys", "Grays", "Roswell Greys", and "Zetans", are alleged extraterrestrial beings whose existence is promoted in UFOlogical, paranormal, and New Age communities, and who are named for their skin color. Around half of all reported alien encounters in the United States describe Grey aliens. Such claims vary in every respect including their nature (ETs, extra-dimensionals, demons, or machines), origins, moral dispositions, intentions, and physical appearances (even varying in their eponymous skin color). A composite description derived from overlap in claims would have Greys as small-bodied sexless beings with smooth grey-colored skin, enlarged head and large black eyes.
The origin of the idea of the Grey is commonly associated with the Betty and Barney Hill abduction claim which took place 1961, although skeptics see precursors in science fiction and earlier paranormal claims. The Grey aliens are also famous from the Roswell UFO incident from 1947.
Source: http://en.wikipedia.org/wiki/Grey_alien. 