Microsoft just announced what its Windows 8 lineup will look like. You're going to have three options to choose from: Windows 8, Windows 8 Pro and Windows RT. But only two of those will be available as upgrades.
Windows 8 will be for most people. It runs on both 32 and 64 bit x86 machines—which is what the overwhelming majority of Windows users have on their desktops right now. Windows 8 Pro, also for x86 computers, is a step up. It includes features like encryption, virtualization and more. It's basically the Windows for businesses.
And then there's Windows RT, which is probably the most intriguing of the bunch. This is Windows for ARM. It won't be available as an upgrade, it's only going to come pre-installed on tablets and other machines with ARM chipsets. It's going to be bundled with Microsoft Office software—Word, Excel, PowerPoint, and OneNote—designed to run in desktop mode on touch screens. (In other words, these won't be chrome-free Metro apps, but will take advantage of gestures and should be tablet friendly.) And in addition to what it does have, it doesn't have Windows Media Player. You won't miss it.
Windows RT is, frankly, the most exciting of all. It's going to mean Windows on more devices than ever before and this is where Metro apps—and the entire Metro experience for that matter—should really and truly shine. [The Windows Blog]