I found this trailer via Superherohype, and despite all the noise on the Interwebs, it appears to make the essentials of the plot known.

What follows below is some analysis, and a bit of conjecture, based upon what has been released so far.

It appears that the beings, represented by “The Space Jockey” from the first “Alien” film, have been to the earth in our distant past and are responsible for humanity, or at the least played a significant role in our development.

They also either created the Aliens (that being said, I am not quite sure why they would create such efficient killing machines.  Then again, we often make all sorts of stuff, from biological to atomic weaponry, so that’s not exactly beyond the realm of possibility) that came to be known as xenomorphs or they are some how a ‘corruption,’ a defect in the natural development of the Space Jockey species (this bit of conjecture is based upon an early trailer) of a room filled with cylindrical pods that looks similar to the ‘eggs’ that birth xenomorphs.

Assuming that the Aliens are a creation of the Space Jockeys, as opposed the the beings themselves, leads to the question of how they got out of their control, since one of their ships was found on LV-426 in the original “Alien” infested (“infested” an apt word because later directors, particularly James Cameron, treated the xenomorphs as little more than very predatory, parasitic, insects; a portrayal that I have always felt showed a lack of imagination) with xenomorphs, which also killed the pilot.

I also think that the crew of the Prometheus become infected by something that they find aboard the alien ship, though whether the “infection” turns them into Space Jockeys (possible, but unlikely) or Aliens (a little more likely) is up for debate.