The third Microsoft-Windows version to add a graphical user interface to MS-DOS, released in 1992. Runs on MS DOS 2.0 or newer (was not an operating system, but a program that ran on DOS). Windows 3.1 introduced better graphic, filetype, and audio/video support to users of DOS, windows 1.0, and Windows 2.0.
quote from Winworldpc.com:
Microsoft Windows 3.1 was an evolution to Windows 3.0 and undoubtably the most popular, poster child version in the Windows 3.x series. Among the changes in Windows 3.1 include a drop of real mode support (see more below), the removal of the Reversi game, updated icons with richer colors, an improved setup process with better hardware detection, and the introduction of batch install. The File Manager was completely revamped and a revamped hypertext help system was introduced.... More Info »
Applications could talk to each other not only through the DDE (Dynamic Data Exchange) protocol, also used by OS/2, but also by the new Windows-only OLE protocol which allows for applications to share any type of object more seamlessly. Write, Paintbrush and the new Object Packager have support for this technology which remains with us today in Windows 8.
Windows 3.1 also came with support for TrueType fonts which provide more realistic font rendering as they are outline fonts that can scale to any point size. With TrueType users could finally have a good grasp that what was shown on the screen would be what was printed without blocky outlines. TrueType survives today along with its close cousin OpenType.
Multimedia support was now fully integrated along with the expandable Control Panel into Windows 3.1. In Windows 3.0 this was provided by a Multimedia PC add-on which usually came with new Multimedia PCs, sound cards and CD-ROM drives of the day. Common supported cards include Adlib and Sound Blaster 16.