The kernel of many Windows OS'. The OS' that use this to date are NT3.1, NT3.5, NT3.51, NT4, Windows 2000, Windows XP, Windows Vista and Windows 7, this excludes server only OS'.
The other kernel was 9x, OS' falling under this are MS-DOS, windows 1.x, Windows 2.x, Windows 3.x, Windows 95, Windows 98 and Windows ME
The NT kernel in Windows is a fine piece of craftmanship.
It's not just used in Windows NT