While Minecraft has long had a "free" mode, that's restricted you to just running around building stuff. The new demo for the game actually lets you engage in the proper singleplayer "campaign", albeit with a time limit.
Players have 90 minutes to run around building, destroying, mining and killing anything they can get their hands on. To help you get a feel for all that's possible with the game, you'll start with every tool, mineral and item to be found in Minecraft. Once the 90 minutes is up the world "locks", but you're free to go and start an unlimited number of new ones.