Microsoft Windows

Microsoft Windows is a family of window frames developed by Microsoft. It currently uses the Windows Explorer shell.

Windows was previously a desktop environment for DOS and now it is an operating system using the Windows NT kernel.

Windows for DOS
During this time, Windows was a desktop environment and display server for DOS, an OS inspired by CP/M. It did include it's own graphics drivers and program API, however it used the MS-DOS API extensively and cannot be considered an operating system.

Windows 9x
In the 1990s, Microsoft developed several versions of Windows where the Microsoft DOS kernel was included and many DOS apps were replaced with Windows ones. Windows was an operating system that used the DOS kernel, essentially a distribution of Microsoft DOS.

Windows NT
Windows NT is an abbreviation to Not Tested