Windows NT
Windows NT is the current line of Windows operating systems developed by Microsoft. This includes:
- Windows NT 3.1 through 4.0
- Windows 2000 (NT 5.0)
- Windows XP (NT 5.1) and Windows Server 2003 (NT 5.2)
- Windows Vista and Windows Server 2008 (NT 6.0)
- Windows 7 and Windows Server 2008 R2 (NT 6.1)
- Windows 8 and Windows Server 2012 (the next version, apparently NT 7, planned for 2012)
...and most likely future versions of Windows as well.
Windows NT does not include Windows 3.x or Windows 9x/Me, none of which were actual operating systems, but rather graphical shells that ran on top of DOS. (Windows NT was developed from the OS/2 codebase.)
For more information, including a more inclusive list of NT versions, see Wikipedia's Windows NT entry.