I just went to
www.playstarbound.com to find out the same thing. From my extremely quick and lazy scanning of some pages, it looks like and "open world" planetary exploration game, where you can build, craft, examine, and do all sorts of fun stuff, solo or co op, all in glorious pixel art. Almost like a more planetary exploration sim vs space sim (and 3d) that Starlight Inception is going to bring us.
Then again, I spent maybe 5 minutes on the site. I'm at work, but I plan on reading moar later.
Edit: After a quick peek at a building trailer it resembles another game in a similar fashion...Terraria I believe, but in space! I'm sure there's plenty of differences between the two titles, just what I was thinking of when I watch the vid.