DOS?! In the 2020s?!
DOS is a very primitive operating system which was the first system released for the IBM PC. DOS is a single-tasking system, which means only one task can run at a time - no, you can't even open a web browser and a text editor at the same time. DOS is also a real-mode operating system, which means programs have full control of the computer. This doesn't sound bad until you realize a malfunctioning program will crash the whole system and require a reboot (and in the worst case, might even destroy your filesystem).
DOS is derived from a system called QDOS, which means Quick and Dirty Operating System - a name appropriate also for its successor. Why would anyone use something like this when even 8-bit CPUs can run multitasking kernels?
The simplicity of DOS means it has very low system requirements and uses very little memory, allowing it to get a lot of out of weak hardware. For this reason, A LOT of games have been released for DOS, ranging from simple 2D games to complex fully 3D titles. Low-level programming for DOS can be surprisingly fun too - you can manually handle the things that are usually handled by the operating system today. On top of that, most modern systems can run DOS software with tools like DOSBox, so your DOS program will be usable on a lot of devices!
DOS tends to work better with older computers, mostly thanks to it being very old and newer hardware being less compatible with low-level details used by DOS software. That's why I'm going to install it as the 3rd system on my Sempron PC!
FreeDOS or MS-DOS?
In the 80s and 90s, when DOS was most used, the most common type of DOS was MS-DOS, which was released by Microsoft. The last version of MS-DOS released as a standalone product was 6.22. There was also MS-DOS 7.0, 7.1 and 8.0, which were only released as integral parts of Windows 95, 98 and ME respectively. MS-DOS 7.1 is popular, as Windows 98 has full DOS support unlike ME and 7.1 supports FAT32 unlike 6.22.
However, MS-DOS isn't the only type of DOS - there was competition back in the day, most notably Digital Research's DR-DOS. MS-DOS is still pretty much the standard of DOS that DOSes from other developers try to be fully compatible with. One of those other DOSes is FreeDOS, which is an open-source system aiming to be fully compatbile with MS-DOS.
I'm going to use FreeDOS myself here, since I like open source software, so why not use it? i might make a post about MS-DOS one day too.