OS/2 was an Operating System developed by IBM. It was originally intended as the successor to MS-DOS; it supported multi-tasking. The first 1.x release supported multi-tasking and Intel 80286 protected mode (providing for up to 16 megabytes of memory). A later release added a graphical user interface (GUI), similar in appearance to early versions of Microsoft Windows. OS/2 also introduced a new filesystem, HPFS, as a replacement for FAT.
Originally IBM and Microsoft collaborated on OS/2, but the relationship unravelled around the time of the development of version 2.0. Version 2.0 introduced a new object-oriented GUI, based on IBM's System Object Model (SOM), and provided a desktop (similar to that eventually featured in Microsoft Windows 95). Version 2.0 was fully 32-bit and could take advantage of the 4 gigabytes of memory provided by the Intel 80386. Microsoft wanted to develop similar features in Windows, instead of OS/2. Microsoft and IBM originally compromised that IBM would develop OS/2 2.0, while Microsoft would develop OS/2 3.0; but the deal then completely fell apart, and Microsoft renamed OS/2 3.0 then under development to Windows NT. Microsoft released Windows 3.1 as its response to OS/2 2.0; but Windows 3.1 lacked most of the important features of OS/2, such as long-file names, a desktop, or being 32-bit. These features were later added to Windows in Windows 95.
OS/2 failed to catch on, and is today little used outside business.
See also History of Microsoft Windows