It's the web browser based version of the full game. You can't save anything you do but it will give you an idea if you will like the game or not.
And you still have to register to play the demo, e-mail, password and the user name of your charactor.
Chat with our AI personalities
No you just log in and play the demo
Go to start menu, type in %appdata%, if there is ".minecraft" when you search, then you can. Anyways, why using demo?
You can play the demo version or Minecraft Classic on the Minecraft website.
To get Minecraft for free, you need to download it on Xbox.
You can play the Minecraft Beta demo by finding the PC Gamer demo on their website. In order to play the full game though, you will need to purchase it from the Minecraft website.