|The woo is out there|
|Aliens did it...|
|... and ran away|
Pleiadians, also known as Nordic aliens, are humanoid aliens that come from the stellar systems surrounding the Pleiades stars, and they're really really really concerned about Earth and our future. (Since the Pleiades are a pretty group of stars and since they're even more beautiful through telescopes, anything connected with them has to be good and kind, right?) So concerned are they, they've contacted certain special people to channel them and convey their message. Which isn't always consistent. The Pleiadians either came from a group called the Lyrans, or coexisted with the Lyrans, or the distinctions between Lyran, Pleiadian, and Sirians don't really exist. Native Americans are of Pleiadian descent, or white people are of Lyran descent, or humans are of no notable relation to either. They can either switch between the third and the ninth dimension, or they exist solely in the fifth dimension (which is apparently made of love and creativity), or just live in non-specific dimensions higher than three. Basically, the only things generally agreed upon by the Pleiadians themselves is that they want to help Earth and let humanity ascend to higher dimensions.
Pleiadians are supposed to be better looking versions of us. They nearly always lack pigmentation in their skin and hair, giving them an albino appearance. Many Pleiadians have an Asian appearance similar to grays. As a rule, Pleiadians don't carry around much excess fat, although females are known to have curvy figures. They don't usually have curly hair or beards.
No particularly elaborate theory is really needed to work out why humans came up with aliens who looked so human. As such, their beauty is reminiscent of African/Egyptian gods and goddesses or angels, who are usually pleasing to look at. "Human" aliens have been a staple of science fiction from the beginning. Erich von Däniken suggests that humans are deliberately designed to look like their makers. In Star Trek, it was easier for actors to fit into human type suits.
Pleiadians and Nordic aliens were a staple of the early contactee (as opposed to abductee) trend of people like George Adamski. Pleiadian statements tie in nicely with the spiritualist and theosophist beliefs of the early twentieth century. They're also a bit like the guy out of the original The Day the Earth Stood Still (he was from Venus though). As the hippie thing died down, and the 1970s became more cynical (Watergate and all), so Pleiadians took a back seat from contact and moved into channelling. The far right likes them as well, and the whole Maria Orsic legend seems tied up with them. (She is said by some to be one herself.)
Most of all, they're the aliens you'd enjoy a nice quiet cup of tea with down at the yoga centre, rather than being eaten and cruelly ruled over (reptoids) or having foreign objects lodged in your rectum (grays).
 New Age claptrap
- For more, see Ashtar
Pleiadians are somehow connected to Atlantis and Lemuria. Reiki, ear candling, Shiatsu, reflexology, aromatherapy, and crystal healing all came from the Pleiadians. The Pleiadians brought dolphins to Earth – we know so because JFK's spirit has contacted a human through automatic writing to tell us so. Jesus was a Pleiadian, as was his father, though his mother was Lyran. They love sex (the Pleiadians do, not Jesus and his family… though they must too if they're Pleiadians). The Pleiadians are helping us fight the evil space reptiles (and you thought we couldn't connect this to Reptoids) as part of the battle against the Illuminati. They are Old, Old, Old Earth creationists, and think Earth is 626 billion years old. Pleiadians are apparently truthers.
According to Barbara Marcinak's Bringers of the Dawn, the common ancestors of both humans and Pleiadians came from another universe (!), and seeded various worlds in our universe (including Earth) with their DNA. Yes, aliens that evolved in a completely different universe, presumably with its own laws of chemistry, have DNA. The Pleiadians appeared before we humans did, and ascended to the next "evolutionary stage" or the Akashic plane or some such nonsense. And wonder of wonders, this moment in history just happens to be the time at which humans are about to reach the same "evolutionary stage" that the Pleiadians already have. And the Pleiadians want to help their "younger siblings."
And the Pleiadians just might have been doing something good and kind in the year 2012 to counter gloom and disaster instead of spreading it. If they did, it seems to have not been something that anyone actually noticed.
 In actuality
The actual Pleiades is an open star cluster, about 400 light-years from Earth. It consists of 7 stars that are visible to the naked eye and a bunch that are too dim to see without a telescope.
The cluster is very young, no more than 150 million years old. Any planets orbiting the stars of this cluster would also be equally young. In fact, when the Earth was 150 million years old, it was still in the Hadean eon, characterized by regular impacts from asteroids and comets. Its surface was largely molten due to the constant, relentless impacts. It would be another two billion years before oxygen first appeared in its atmosphere.
In other words, if space aliens did evolve on any of the (hypothetical) worlds in the Pleiades cluster, they'd be the equivalent of bacteria.