Many of you will at least know os/2 as the operating system jointly developed by IBM and Microsoft. The two companies started their cooperation in 1985 with the Join Development Agreement, and the first version of os/2 was the first result of this agreement. The name of the operating system is derived from the new computer IBM was shipping, the ps/2 (personal system/2); I actually have an old ps/2 somewhere in the attic at my parents' house.
The first version of os/2, released in April 1987, was a text-mode only operating system, lacking the graphical user interface ('Presentation Manager') due to time constraints. Despite this missing feature (and others that were missing as well), os/2 still meant a fundamental departure from the DOS era. It had many features DOS did not have, such as:
Interestingly, os/2 1.0 also supported running DOS programs. However, you could only run one DOS application at a time, due to a number of hardware constraints. The DOS program had to run in 'real' mode, an operating mode for x86 processors in which software had direct access to BIOS routines and peripheral hardware; multitasking and memory protection are not supported, and there is a limit of 1MB of usable memory. Since os/2 ran in protected mode, switching between real and protected mode was needed. The DOS program could not run in the background; it had to run fullscreen. Your os/2 applications kept running in the background, though.
The Presentation Manager graphical user interface, missing from os/2 1.0, was introduced with the release of os/2 1.1 in November 1988. As with the rest of os/2 at that time, it was jointly developed by IBM and Microsoft, and you can surely see the resemblence between PM and the Program Manager later found in Windows 3.x.
In the early 1990s, the collaboration between Microsoft and IBM started to crack. Microsoft scored a tremendous hit with Windows 3.0, which came bundled with many new computers, while os/2 was an expensive stand-alone package. In addition, os/2's hardware support was limited; for instance, only a few IBM printers were supported, no HP, no Epson, nothing. Windows' hardware support was much better during that time, and that certainly contributed to its popularity. The popularity of Windows made Microsoft shift its focus away from os/2 and IBM to its Windows platform. Wikipedia gives a detailed overview of the reasons why the two companies parted ways.
Because of the drawbacks in os/2 during that time, Microsoft hired Dave Cutler to start working on a version of Windows, a version that would be more portable and more future proof. This version would be Windows NT, and early versions of Windows NT contained specific bits and pieces of os/2 code, such as support for os/2's HPFS filesystem, running os/2 1.x text-mode applications, and, most notably, the os/2 LAN Manager. Up until Windows 2000, NT had an os/2 subsystem for running os/2 text-based applications.
IBM continued to develop os/2 by itself, which resulted in os/2 2.0 in 1992. os/2 2.0 Was a 16/32bit hybrid operating system, and it supported running multiple instances of DOS and Windows 3.x, thanks mostly to the virtual 8086 mode of the 386 processor. Windows 3.x applications could be run 'seamlessly' (as if they were os/2 applications), or on a either fullscreen or 'windowed' Windows 3.x desktop. os/2 2.0 Also saw the introduction of a new graphical user interface: the Workplace Shell, an object-oriented desktop shell.
1994 Saw the release of os/2 3.0, which retained its codename 'Warp'. Warp 3 improved upon os/2 2.x in many ways; the printer/display driver models were improved, and in general, Warp 3 had a much larger driver pool than its predecessors. On top of that, multimedia support was improved. Visually, Warp 3 received an overhaul, with new icons and more pleasing colour schemes. On the network front, Warp 3 had better support for the internet, while also coming with a basic office suite, IBM Works.
In 1996, IBM released the last major release of os/2, known as os/2 Warp 4. This release added support for Java as well as speech recognition. IBM continued to feed os/2 users with point releases, and the last official IBM release was os/2 Warp 4.52, in December, 2001 (OSNews was a little late to the game). Official support for os/2 by IBM ended 31st December, 2005.
However, that was not the end for os/2. IBM allowed Serenity Systems to continue development of os/2; this version became known as eComStation (eCS for short). Its first release was on 29th September, 2000. Serenity Systems works together with IBM and several other 3rd parties to develop eCS. Even though some believe this situation is similar to that of Zeta and BeOS, this is absolutely not the case. eComStation is fully authorised by IBM, and is completely, 100% legal.