(This has been so useful. I typed it up for a listserv a while back, and it helps to reference to keep track of what MS's wacky names for Windows really mean)
This is what happens when people stop using version numbers. Anyhow, heres a breakdown of Windows version numbers and their names:
Windows based ones (Created by MS alone, client OS)
Windows 1.0
Windows 2.0
Windows 3.0
Windows 3.1
Windows 4.0 (95) - Dos 7.0 included
Windows 4.1 (98) - Dos 7.1 included
Windows 4.9 (ME) - Dos 7.? included
Windows NT based ones, aka started from the OS/2 code shared with IBM, mainly used in the corporate enviornment for now:
Windows NT 3.1 (3.1 to match Windows 3.1)
Windows NT 3.5
Windows NT 4.0
Windows NT 5.0 (2000)
Windows NT 5.1 (Codename Whistler, real title Windows XP, still in beta)
Windows 3.1 was replaced with Windows 3.11. Also there was a Windows for Workgroups 3.1 and 3.11. These were the versions to go with NT 3.1.
Windows 95 had a few revisions seen only by OEM's, usually referred to as OSR 2, 2.1 2.5 etc...
Windows 98 also had a Second Edition, quielty replacing 98 on the shelves. The version number was the same except the build number (not shown in the chart)
Also, MS intended 2000 to be the replacement for 98, but backed out of that in beta. They decided it wasn't quite client oreinted yet, but Whistler is slated to fix that. (2000 is perfectly capable of being a gaming/media OS, just MS Marketing doesn't want you to know that)
And lastly, ME is just all the patches for 98, icons from 2000, and DOS mode hidden plus Media player 7 and other features like that. All Windows (non NT) OS's run on top of DOS.