The Windows NT line of operating systems was a completely new line of operating systems developed by Microsoft in parallel with the MS-DOS/Windows 95/98/ME line. Windows NT was for business; Windows 95 for home users.
They hired a group of developers from Digital Equipment Corporation who developed the VMS operating system. Consequently there are many similarities between VMS and Windows NT.
Windows 2000 (Windows NT 5) was the first version where MS ceased to market the two separate product lines and release a single operating system for both home and business users.
Windows XP (Windows NT 5.1) was a minor evolution of Windows 2000. The main difference was the change away from the serious business-looking interface to a more game system/toy feel which appealed to a wider user base.
Windows Vista (Windows NT 6) is the latest evolution of the Windows NT line.
Windows NT has nothing to do with DOS or FreeBSD. It has more in common with VMS as Microsoft poached the VMS development team from DEC to develop it.
The history of Windows NT is well documented on Wikipedia:
http://en.wikipedia.org/wiki/Windows_NT