In spite of the common usage, none of these systems were named simply "DOS" (a name given only to an unrelated IBM mainframe operating system in the 1960s). A number of unrelated, non-x86 microcomputer disk operating systems had "DOS" in their name, and are often referred to simply as "DOS" when discussing machines that use them (e.g. AmigaDOS, AMSDOS, ANDOS, Apple DOS, Atari DOS, Commodore DOS, CSI-DOS, ProDOS, and TRS-DOS). While providing many of the same operating system functions for their respective computer systems, programs running under any one of these operating systems would not run under others.
DOS systems first ran on Floppy disks. There were two types of Floppy disks DOS systems were on, 3.1" and 5.1". Since the size was small (Up to but not exluding) 2MB. DOS systems were very basic and ran on simple commands. The two most known operating systems that people recognize first are Windows 1.0 and Macintosh OS. There were two types of DOS. GUI DOS-Based-Systems and actual Command Prompt DOS systems. GUI DOS-Based-Systems are most fondly remembered for being "revolutionary" for their time. They modernized the DOS system and after the GUI DOS-Based-Systems came the Windows we know and (possibly) love. But the first DOS systems some people remember mostly is MS-DOS and Basic. These were the Command Prompt DOS systems.
There are many DOS viruses out there, the first being Brain, though there are some more notorious ones like CMOSDead. While some DOS viruses doesn't work on Windows or other operating systems, if it is run on Windows 9x, it may cause the system to be unstable and cause multiple Blue Screens of Death.