The free version is the demo - it includes the first 3 islands. The paid version has a lot more content (about 4x the length), and multiplayer.
I know that, because I completed the demo-it took me 3 days and 4 something hours!!! At the end of the demo you find out about the others planets and Unoiks 428 or something like that. Then it shows you the credits. I was really sad because I wanted to play the rest of the game. I'm only 10 so I could not buy the full game. I wish it was free, although great game!!!😄