Noder's note: I cannot say I believe any of this, but I have been reading about this subject for years, so I should be reasonably well versed.
They are probably the most well known type of alien. Greys are the staple alien race of abduction claims/fiction. They are about as old as the UFO phenomenon of modern times, which is generally accepted to have started in June 1947 when Kenneth Arnold saw flying crescents (yes, crescents, not discs) over Mount Rainier. Common pictures of aliens from the Roswell Crash (also 1947), arguably the most famous UFO-event, show Grey-type aliens.
So the Greys are famous, but what do they look like? They are small - between three and four feet in height, with spindly limbs and fingers. Their heads are large, with disproportionably sized craniums. They have large, entirely black, almond-shaped eyes, noses that are just two nostrils, and only a slit for a mouth. They have grey skin and wear tight fitting grey one-piece suits, which leads to their name. UFO literature speaks often of greys having atrophied digestive systems, leading Greys to receive nutrients through their skin.
The Greys are nasty. Some aliens abduct people to preach universal love, or some such, to them. Not these bastards. They'll have you on an uncomfortably cold gurney, naked, as soon as look at you. Greys seem to enjoy performing medical experiments on humans and removing vital parts from cattle. The Greys are also well known for building underground bases (possibly to store all of the cow parts) and being in league with the New World Order / U.S. Shadow Government.