You should read the main game store page for the full description of the game.

In the full game, you have access to the three levels of the manor, while in the demo, you are limited to the area outside of it and it finishes as soon as you've read the "message on the door."

The demo itself does not have any other limits. You can launch it as many times as you want, scores and achievements are working, and they will be transferred to the full game if you decide to go for it!

In terms of what to do, well, most of the action happens inside the manor, but there are still quite a few things to inspect, items to collect and combine, and at least three different ways to reach the game over sequence. The full game just has more of that, so the demo should give you a pretty good idea of what to expect.