Questions with this tag should refer specifically to versions of Windows 8. Anything related in general (non-version specific) to Windows OS should be tagged with windows.
An older publicly available version of Windows 8 is the Windows 8 Enterprise Evaluation, but the latest non-trial version of Windows 8 is the Windows 8 Release Preview. In addition, MSDN and TechNet subscribers have had access to Windows 8 RTM releases since August 15th.
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) 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 use with mice and keyboards. A new "Start screen", similar to the one in Windows Phone 7, includes live application tiles. The Start screen replaces the Start menu, is triggered by the Start button on tablets or the Windows key on keyboards, and is also the first screen shown on startup. The user can go to the regular desktop by choosing the "Desktop" tile or a traditional desktop-based application.
The new applications run in full-screen, but two of them can be displayed side-by-side using "Snap". Examples of new applications that were demoed 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 ARM processors, in addition to continuing support for traditional x86 processors.
Significant client side technologies from Windows 8 have been incorporated into Windows Phone 8, helping to ease porting applications between these two distinct platforms.
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.