One of the niceties of Microsoft's upcoming Windows 8 operating system is that the number of "editions" of the OS has been reduced from six to four, with much of the consolidation coming at the consumer end of the spectrum.
When considered in light of the traditional Windows desktop model, however, Windows 8 really only has three editions, with new entrant Windows RT rounding out the offering. Windows RT is the final name for what was commonly called "Windows for ARM," Microsoft's attempt to bring Windows to ARM-based processors commonly found in mobile devices.
Why ARM? - ARM Holdings, the company behind so-called "ARM CPUs," has been around for decades, originally producing processors for low-power devices like networking equipment and consumer electronics. The design's low power requirements and relatively open licensing of the technology quickly found ARM-based processors powering the world's mobile phones. Companies like Qualcomm and NVidia developed "system on a chip" packages, where core components of a smartphone — like the CPU, video processing, and mobile network hardware — were contained on a single chip, further reducing power consumption.
Contrast this with Intel's x86 design, common to traditional desktops and laptops, which was originally designed with little concern for power consumption or space savings. This rarely presented any challenges to Intel until the smartphone and tablet era took hold, leaving Intel in catch-up mode while ARM-based designs dominate tablets and smartphones, with Intel "inside" a grand total of three smartphones as of this writing.