Thanks for your interest! The difference is that you can only access about 2 areas of the game, with the first boss included in the demo. Also max. level is only 5 instead of 30, which means that the dragon cant evolve fully. That's basically all the changes there are.
You can also use the save data from the demo (sav.arr) in the main game if you decide to get it, and continue where you left off.