The discussion of what kinds of aliens exist is a pretty broad spectrum. The variety of alien types that people have theorized about, ranges from Jabba the hutt-esque beings, to the alien in Alien vs Predator.
Some widely talked about and accepted (theorized) types of extraterrestrial are as follows.
This type is the common, almost stereotypical kind of extraterrestrial that comes to mind when someone says ‘alien’. Greys are described as thin, bipedal, having large oblong heads, and long limbs. They can be anywhere from 3 to 6 feet tall, and have a weight ranging from 50-90 pounds. A Grey’s facial features (which corresponds immensely with Hollywood’s typical alien) features large, unblinking, and oblong eyes, a relatively small mouth resembling a crease or slit, and very small to nonexistent nostril holes.
The Grey’s means of communication is thought to be nonverbal (hence a small or even nonexistent mouth) such as telepathy.
Their agenda is theorized to be generally scientific studies and medical research (mainly of the human race). Greys were the type in the 1947 Roswell UFO Incident, which would account for most alien sightings and abductions to be (supposedly) linked to Greys.
The Reptilian type is bipedal, stands 6-9 feet tall, has scaly skin, and a facial structure with ridges and a protruding “lizard-like” jaw.
This particular type of extraterrestrial has many theories backing it, pertaining to their relation to the human race and Earth. Some suppose that the Reptilians “seeded” Earth and thus created the long reigning reptiles, some of which are still seen today (ie. alligators, crocodiles, Gila monster, etc.).
The Reptilian’s agenda has been theorized to mainly be dominance over varying races, whether it be Grey aliens or humans. “Believers in this theory claim that the Reptilians live in the 3rd, 4th and 5th universal dimensions, feed on negative energy, and attempt to cause conflict in our world,” (Michelle).
Nordics are a very human-like extraterrestrial, also called Blondes and stand 5-7 feet tall. Abduction accounts claim that this type always has blonde hair, and most commonly blue eyes, though occasionally violet, pink, red, purple, and green. Nordics can easily pass off as a human from a moderate distance.
Their means of communication are theorized to be through telepathy telekinesis. Their purpose is theorized to be “silent watchers of Earth” that aid in helping the human race if needed and warning about potential dangers in the universe.
Michelle, Caroline. “Do Aliens Exist? Three Prominent Alien Types Exposed.”HubPages. N.p., 2010. Web. 26 Feb. 2015.