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 & 5.2)
• Windows Vista (NT 6.0)
• Windows 7 (NT 6.1)
• Windows 8 (the next version, planned for mid-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.

For more information, including a more inclusive list of NT versions, see Wikipedia's Windows NT entry.