A microcomputer operating system from Microsoft, written for the original IBM PC. MS-DOS was based on 86-DOS, an OS written by Seattle Computer Products for a line of systems they sold based on Intel 8086 processors running on an S-100 bus. It was single-user and single-tasking, and introduced the FAT file system for disks.
MS-DOS was issued separately though version 6.22, after which it was supplanted by Windows version 3.X. Windows 3.X was a multi-tasking shell on top of DOS. Windows 9.X was a protected-mode graphical OS that used DOS as a real-mode loader. Windows NT, 2K, XP and Vista are fully protected mode OSes, and include a "Virtual DOS Machine" for legacy support.