MS-DOS (Microsoft Disk Operating System) was a text-based, single-tasking operating system originally released in 1981 for the IBM PC. Early versions of Windows (from Windows 1.0 in 1985 through Windows ME in 2000) were not standalone operating systems but graphical shells that ran directly on top of the underlying DOS foundation.
The functionality and architecture of this classic era can be understood across their core technical capabilities: Core Functionality of MS-DOS