The successor to Microsoft Windows 7 that focuses on a new interface style for touch-based devices and tablets.
Windows 8 is the successor to Microsoft Windows 7. Windows 8 is another addition to the Windows NT family.
The "Windows 8 User Interface," formerly named Metro, has been designed to make it better suitable for touchscreen input in addition to the traditional mouse and keyboard input. The new design focuses on tablet and mobile integration. Applications targeting the new UI (windows-store-apps) can be hosted in Windows Store (windows-store) and submitted via the visualstudio IDE. These must be developed using windows-runtime and either winrt-xaml, html+winjs or directx (DirectX can be mixed with XAML).
Windows RT (not to be confused with WinRT or Windows Runtime) is a version of Windows 8 exclusively for arm microprocessors that only supports the new user interface developed with windows-runtime, while Windows 8 for the x86 microprocessors from Intel and AMD also supports traditional desktop user interface developed with technologies such as winapi, wpf or silverlight.
A full description of Windows 8 can be found on Wikipedia.