Stands for Disk Operating System. Although DOS is a general term used to name different operating systems, often after adding a prefix, the most famous DOS is without doubt MS-DOS (Microsoft DOS). MS-DOS had been the most widely used operating system on IBM-PC compatible computers until Windows 95 arrived, which was intended as a replacement. DOS is a command-driven operating system, and you might appreciate the versatility of DOS commands (how can you erase all the files in a directory having the .TMP extension with a single action? Answer: DEL *.TMP). You can still run DOS commands inside Windows and run DOS applications however Windows NT based operating systems such as Windows 2000 do not give full support to DOS.