Windows 7 is the latest edition in a series of desktop operating systems and graphical user interfaces (GUI) from Microsoft. Windows 1.0 was released in 1985 as a GUI that sat on top of Microsoft’s popular DOS disk operating system. Over the years Windows has been changed and refined, finally subsuming DOS and becoming a full operating system in its own right with the launch of Windows 98.
Windows 7 was released in October 2009. It is not exactly the seventh version of Windows. Rather it is the seventh version from its particular branch of the software. There have been two branches of Windows: the original consumer versions and the New Technology (NT) business versions. The original consumer lineup included the popular Windows 3.1, Windows 95, and Windows 98. It ended with Windows Me. The NT series began in 1993 as an offshoot of Windows 3.1, with much of the underlying code reengineered to make it more stable and suitable for business users. This NT development tree has split off further with the Server, Desktop, and Home Server variations of the operating system. In turn, the NT branch spawned various server versions of the operating system and then Windows XP, Windows Vista, and most recently, Windows 7.
There is some debate about whether Windows 7 really is the seventh iteration of the NT family, but it’s not the most important concern facing the world today. Windows 7 is officially the seventh iteration if you follow the tree Windows 1.0, Windows 2.0, Windows NT 3.1, Windows NT 4.0, Windows XP, Windows Vista, and Windows 7. Depending on your view, there have been as many as 28 versions of Windows since it first launched, up to 2010. Windows in its various versions is currently used by approximately four billion people worldwide.
Source of Information : Microsoft Press - Troubleshooting Windows 7 Inside Out