Disk Operating System, often also known (though not exclusively) by the name MS-DOS; a version of DOS bought from Gary Killdall by Microsoft Corporation to be sold to IBM in implementing the latter's IBM Personal Computer.
DOS was an extremely primitive OS by today's standards, and in comparison to existing OS's at the time. However, through such small measures are billionaires made. It was easier and quicker for IBM to buy something than to build it from scratch, and those at IBM attempting to get the thing to market had bigger problems to deal with at the time, including internal hostility towards the technology.
There are some free DOS implementations. One such implementation is DOSBox, which is primarily designed to run old DOS games. It can be installed using the command:
# apt-get install dosbox