What is the Autodesk 123D app?

Autodesk announced, with quite some vagueness, 123D, as an entry-level, free design tool with links to "make" things (reading: fabrication).

It's unclear what it is, precisely, so no clue if this is Windows, OSX, iOS, Android compatible. It is said to be free and also to be some kind of entry into the range of Autodesk software solutions.

I hope it is easy to use, yet sophisticated enough. There are so many complex CAD applications and even SketchUp is now quite extensive and overly crowded with tools, toolbars and add-ons.

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Currently, it has become clear that 123D is a free, Windows-only version of Autodesk Fusion, which in itself is based on Inventor, their MCAD offering and is also included with AutoCAD 2012.

It aims to provide an Autodesk alternative against SketchUp (and maybe Bonzai3D) for advanced, yet simple to use modeling.