The DOS (disk operating system) is generally called the PC-DOS operating system, as well as the MS-DOS variant sold by Microsoft for PC compatibles. There are later clones, such as DR-DOS from Digital Research and FreeDOS.
Until the early 1990s, DOS was the most used type of system on PC compatibles. This command line system was rudimentary: no multitasking, no virtual memory, management of the only 16-bit segmented mode of the x86 microprocessor.
There are other unrelated systems that contain the word DOS (AMSDOS, AmigaDOS, Apple DOS, ProDOS, DOS on mainframe), but their names are overshadowed.