Appart from what was shown in the Mario Maker trailer, we know not much about the game. But it will let you create levels in the good old NES 8-bit Super Mario Bros.-style. And i looks very fun. You can also change it to the look of New Super Mario Bros. U-style on the fly.
But I hope and quess that the levels can be uploaded for others to download. It looks very promissing.
Mario Maker will be out on Nintendo Wii U in 2015.