an operating system made through a joint venture between microsoft and ibm. it was the predecessor to WinNT. it could run dos, win16, win32, posix as well as os/2 native apps. it really was an amazing operating system at the time with a VERY passionate community behind it. watch some of the videos for a good take on it:
i learned about OS/2 from the 'Doing Windows' series. really recommend it, it's a great read about the history behind all this stuff, the computing landscape of that period, and why OS/2 was a huge achievement.
the road to "run DOS stuff [without being DOS]" was very long, and paved with many gravestones... i think OS/2 comes in in part 5 or 6, but i really recommend reading the whole thing.
tl;dr A graphical OS developed by IBM that succeeded DOS and competed with Windows. Notably, it featured pre-emptive multitasking before Windows did. It was not a success in the home market but was reasonably successful in big business, especially finance, for a short amount of time.
As a tween/teen, I learnt a lot from OS/2. Up until then I had only used DOS and Windows 3.x. And then my Dad bought me a copy of OS/2 2.0, and also the Walnut Creek Hobbes OS/2 CD-ROM. And I discovered EMX (the OS/2 equivalent of Cygwin). And I started playing with bash, EMACS, GCC, etc. Next thing you know, I was installing Slackware Linux. At which point I largely lost interest in OS/2. But EMX was an important stepping-stone for me in getting in to Linux.
I think it's important to note (even in a tl;dr) that for a time OS/2 was a joint venture between IBM and Microsoft, and that MS sabotaged that relationship while secretly working on WinNT.
On a related note, "Showstopper!: The Breakneck Race to Create Windows NT and the Next Generation at Microsoft" is a surprisingly entertaining story, and reads more like a novel than a documentary/memoir.
As a result of a feud between the two companies over how to position OS/2 relative to Microsoft's new Windows 3.1 operating environment, the two companies severed the relationship in 1992 and OS/2 development fell to IBM exclusively.
Windows 3.0 was eventually so successful that Microsoft decided to change the primary application programming interface for the still unreleased NT OS/2 (as it was then known) from an extended OS/2 API to an extended Windows API. This decision caused tension between Microsoft and IBM and the collaboration ultimately fell apart.
It was the days where people owned their own software and DRM had not made it's way into games, since the internet has enabled PC game theft on a massive scale, by valve, ea and activision.
OS/2 was an alternative Operating system oriented towards businesses that could run apps from different operating systems under one unified framework.
