It is set in a time before Alien but in the same universe, featuring the Weyland company and other elements of the Alien films. Ridley Scott actually had the idea for a long time, but other projects kept getting in the way. So yes, technically it is a prequel. However, the director and writers are trying to get away from the idea that it is just that.

A typical prequel is very closely tied in with the original work - its main aim is to tell you 'what happened before the story starts', often making things clear that were not explained in the original. Prometheus is meant to be a lot more standalone, with its own themes, storyline and ideas; so if you are expecting it to fill in all the gaps left by the Alien films you may be disappointed. Therefore, the publicity is not promoting it as a prequel - you should be able to enjoy it even if you haven't seen Alien.