Mario Maker is indeed real, as you may already have seen in one of the many, many leaks before Nintendo's Direct presentation.
It's been shown off by Nintendo and allows you to take original Mario levels - from the original and NSMB series - and rearrange, mess about with and generally edit.
It's a level editor, basically. But a level editor with Mario. We've got silly excited about level editors for two games in the last two days now.
Man, level editors are great.