OS/2 is an operating system from IBM. The latest release is OS/2 version 5.
The OS/2 was original developed by IBM and Microsoft. And it was supposed to replace DOS as the operating system on the PCs. Most notable for the user it has a Graphical User Interface (GUI). The first really usable version for none-computer-professional users was OS/2 2.0. Now the operating system had grown up. The user interacted with the computer through Presentation Manager. The file system was HPFS - High Performance File System (2 MB cache on workstation, unlimited in HPFS386). The filesystem was installable (IFS), so when the CD-ROM players appeared it was only a new IFS (CDFS). The operating system handled virtual memory, so expensive RAM could be saved by inexpensive harddisks.
IBM implemented many features from SAA (Systems Application Architecture) in OS/2. Wich means OS/2 includes REXX computer language, SNA networking (System Networking Architecture) through Communication Manager, database, CUA (Common User Access) and a lot more.
The first versions of OS/2 had enormous requirements on hardware (at the time). This made the users to proceed with DOS instead of migrating to OS/2. When Microsoft realized this and when IBM and Microsoft had different oppinons whether to use 386 or not Microsoft broke up with IBM and countinued to work on Windows.
Microsofts part in OS/2 had mainly been the user interface and compilers / linkers. At least OS/2 Warp3 (version 3) still contained a linker copyrighted to Microsoft. Microsoft keept on working on their version of the operating system. And when OS/2 version 2.0 had been released the next number was 3.0. So when Microsoft released Microsoft Windows NT it was version 3.0. Supporting OS/2 version 1 programs, and HPFS.