Ancient Microsoft Disk Operating System, a more modern open source version is FreeDOS.
Ranked in these QuestionsQuestion Ranking
Pro Brings back memories to older developers
Pro Best OS to run QBASIC on
While QBASIC works on newer operating systems, such as Windows 95 and Windows 98, it was designed for and runs best on MS-DOS.
Pro Use old editor Edit
The grandfather of Notepad, very easy to use, hassle-free text editor.
Con Not a modern OS
If you need to know how to run legacy software that will run on DOS (crazier things have happened, there's probably still some ancient, leviathan, software out there that requires it), go for it. Otherwise you're practicing skills that are out of date and are using an operating system that is woefully insecure.
Con Literally horrible
MS-DOS is terrible -- just read about real mode. It was an almost decent solution for its time, but not anymore.
Con Way too simple
A stripped down version of Unix, some commands just got renamed and advanced options removed.
Con No internet support
Using Internet with MS-DOS is not trivial.