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.
https://en.wikipedia.org/wiki/OS/2
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.