From Rosetta Code

Windows is a generic name for a series of loosely-related operating systems developed by Microsoft. The earliest versions, through Windows 3.2, were 16-bit shells that ran on top of DOS rather than true operating systems, while the 9x line (Windows 95, Windows 98, and Windows Me) were 32-bit shells on top of DOS. The Windows NT line of operating systems (starting with Windows NT 3.1 and continuing through the current version, Windows 10) has no ties to DOS or the 9x line but was instead developed separately.

Not to be confused with the windows that are the basis of nearly all modern graphical user interfaces (GUIs). See Wikipedia's "window" article for a thorough discussion of what a window is.