Windows 8 is the name for Microsoft's next version of the Windows operating system. During the //build/ conference in September 2011, Microsoft released a developer preview version of windows-8 to let developers come up to speed with the new Metro UI look and design philosophy (previously seen in Windows Phone 7, Zune HD, the Xbox Dashboard, & Media Center) and it's new underlying runtime: winrt.
The main feature that was shown is the extensively redesigned user interface, optimized for touch as well as for use with mice and keyboards. A new "Start Screen", similar to the one in Windows Phone 7, includes live application tiles. It replaces the Start Menu, being triggered by the Start button, Charms bar, or click in the lower-left-hand corner, and is also the first screen shown on startup. The user can go to the regular desktop by choosing the "Desktop" tile, a traditional desktop-based application, or pressing "Win + D".
The new applications run in full-screen, but two of them can be displayed side-by-side using "Snap". Examples of new applications that include a Twitter client, a weather application, a stock-tracking application, an RSS news feeder, and a virtual piano.
The new interface is primarily designed for 16:9 screen resolutions, with 1366×768 and larger screens able to display two Windows 8 applications. 1024×768 screens can display one Windows 8 application in full-screen, and 1024×600 screens can only use the traditional Windows desktop.
Windows 8 also adds support for the first time for ARM processors (through the new Windows RT edition), in addition to continuing support for traditional x86 processors.
Mike Angiulo confirmed at Computex 2011 that Windows 8 will use OEM Activation 3.0 instead of Windows 7 OEM Activation 2.1, which will make it less prone to cracks.
Since the Developer/Consumer/Release Preview is pre-release code and not necessarily representative of the the questions people with ask about the release version, any questions specific to the Windows 8 preview should be tagged as windows-8-preview.