NT was a parallel line of “professional” windows. It had a different kernel or something. There were equivalent versions to most of the home releases.
The first release was NT 3.1, to match version numbers with the home OS.
NT 4 was the professional version of win 95/98.
In the year 2000 Microsoft released both Windows ME, and Windows 2000. ME for the home, 2000 was the NT release for the workplace.
The products were merged with windows XP, now all windows is windows NT.
The version numbering makes sense if you count by the NT version numbers. 2000/ME is version 5, therefore XP is 6, and if you pretend Vista never existed (as you should for your own sanity) you get to windows 7 and it all starts to make sense.