Windows 3.1 was the 4th version of Microsoft’s GUI-based operating system, released in 1993.
3.1 was a paid sub-release -- Windows 3.0 had been so popular that Microsoft decided to charge for the modified software rather than offer it as a free update to the existing version.
Windows 3.1 retained the same graphics as windows 3.0 with 16 colors aside from some small aesthetic changes. Windows 3.1 features included support for TrueType fonts and peer-to-peer networking.
The new networking capability was one of the main factors in the emergence of the client/server structure that we still mainly run on today, both in home and business networks as well as on the Internet at large.
See a video walk-through of Windows 3.1: