DOS : First successful operating system for personal computers, developed by Microsoft and licenced to IBM for use on the IVM PC. Later on, IBM and Microsoft developed each their own version of DOS : PC-DOS for IBM, MS-DOS from Microsoft. But they are largely compatible.
Microsoft then developed Windows as an alternative to the Apple Macintosh operatingh system and it's remarkably user-friendly interface. At first - up to Windows 3.11- Windows was not much moere than just another interface for MS-DOS, but from Windows95 on, Windows became more of a real operating system, and Microsoft tends to discourage use of DOS, although it can still be very helpful to solve Windows problems. Getting to know DOS
Some people liked DOS, and/or don't like Microsoft's tendency to monopoly in software, or want to be able to still use computers that con't run the heavy Microsoft Windows applications - so they decided to build theuir own DOS.
aka BAT files : creating simple programs/scripts in DOS