You start up Windows as if it were a normal application program running under MS-DOS. But as Windows loads, it becomes almost a full-fledged operating system. It's not quite an operating system because it runs on top of MS-DOS. While Windows is running, it shares responsibility with MS-DOS for managing the hardware resources of the computer. Basically, MS-DOS continues to manage the file system, while Windows does everything else. Windows commands the video display, keyboard, mouse, printer, and serial ports and is responsible for memory management, program execution, and scheduling.
Windows is strong where MS-DOS is weak, and weak where MS-DOS is adequate. Windows includes almost no support of file I/O, which is one of the most essential chores of a minimal operating system such as MS-DOS. This leads to some amusing—or not so amusing—consequences. It is easier in a Windows program to create a disk-based metafile containing a complex series of graphics drawing commands than to create a simple ASCII text file. The former is a Windows job; the latter requires that the program use MS-DOS.