Out of all the products Microsoft is going forward with, Windows RT is undoubtedly the biggest mess. Xbox continues to see strong sales, Surface packs impressive hardware, Windows Phone 8 is gaining traction, Office is soon to see a new release and Windows 8 is selling millions of licences. But Windows RT? It's all over the place.
It's not surprising that Microsoft wanted to bring Windows to ARM-based devices, after all, the iPad is doing so well and Android already has a wide range of devices on the market. ARM-based devices can be cheaper and more accessible than their x86 counterparts, allowing more consumers to enjoy Microsoft software in the home and office.
The product Microsoft came up with is Windows RT: an OS that looks and feels like Windows 8, but really isn't. Sure it may have a very similar code base, and very similar features including the Start screen and the desktop, but there is a fundamental difference between the two. As Windows RT is for ARM devices, and Windows 8 is for x86 devices, apps for one cannot work on the other without recompiling. This means that the huge x86 Windows desktop app catalog is incompatible with Windows RT.