DOS is something of a precursor OS to Windows from Microsoft. It's still used in a few specific areas, but for most of the world, it ceased to exist well over a decade ago.
You could probably still find some "DOS For Dummies" style books out there if you looked, and then there is the FreeDOS project which aims to create as close to a 100% accurate recreation of DOS 6.22 (the last official version from Microsoft) that you could load into a VM or something.
A much more useful thing to learn would be Linux. If you can navigate a Linux bash prompt (like a DOS prompt interface) then you'd have a skill employers might actually want. Depending on what kind of work you're in (or looking to get into) of course.
Ah yes. The glory days of PCs when 8-bit OS ruled and game had an astounding 4 colors and high fidelity digital sound was a collection of beeps. Yeah... if only EA would release a DOS version of Battlefield 3... that would be awesome. EA would truly dominate the gaming industry for decades to come.
Hey I wouldn't say it was that bad! We had 256-colour VGA, MIDI music and digital audio before gaming moved to Windows 95. You can't say Doom, Heretic, Duke Nukem, Dark Forces, Syndicate Wars, Flashback, Lemmings, Worms, Little Big Adventure etc weren't fun.
You also have to remember that back in those days programs had direct and full access to your hardware; none of this DirextX/API stuff; so they ran at blazing speeds... and when they screwed up they could put millions of crap to your printer or HDD in a blink. LOL