Also see Disk Operating System.
MS-DOS (Microsoft Disk Operating System) was the Microsoft-marketed version of the first widely-installed operating system in personal computers. It was essentially the same operating system that Bill Gates's young company developed for IBM as Personal Computer - Disk Operating System (PC-DOS). Most users of either DOS system simply referred to their system as Disk Operating System. Like PC-DOS, MS-DOS was (and still is) a non-graphical line-oriented command-driven operating system, with a relatively simple interface but not overly "friendly" user interface. Its prompt to enter a command looks like this:
C:>
The first Microsoft Windows operating system was really an application that ran on top of the MS-DOS operating system. Today, Windows operating systems continue to support DOS (or a DOS-like user interface) for special purposes by emulating the operating system.
In the 1970s before the personal computer was invented, IBM had a different and unrelated DOS that ran on smaller business computers. It was replaced by IBM's VSE operating system.