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 8 (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.