DOSThe colloquial name for MS-DOS, an early operating system produced and sold by Microsoft. DOS stands for "Disk Operating System", and while other OSes have used this acronym in their name, the popularity of MS-DOS allowed that OS to essentially co-opt it as their own. If you hear someone say "DOS" they almost always mean MS-DOS.
MS-DOS was based on a CP/M OS from the now-defunct Seattle Computer Products for 8- and 16-bit Intel CPUs to satisfy IBM's search for an OS for their new personal computer, and enjoyed widespread popularity for more than a decade, eventually being subsumed into 32-bit versions of the company's Microsoft Windows operating systems. IBM sold a version of MS-DOS with minor differences under the name PC-DOS, and third party clones were sold from time to time with varying degrees of success (in some cases prompting legal action from Microsoft).
There exist many of DOS emulators for non-DOS systems. For instance, Linux has quite a good free DOS emulator running on i386 architectures (also runs under *BSD). But it is actually a hardware emulator, and requires that you use an actual DOS software (e.g. DR DOS or FreeDOS) to run on top of the virtualized hardware. Newer versions of DOSEMU will be released as a bundle with FreeDOS.