Short for WIN32 subset, a software package that can be added to Windows 3.1 and Windows for Workgroups systems to give them the ability to run some 32-bit applications. As the name implies, Win32s is only a subset of the Win32API used by Windows 95 and Windows NT. The main function performed by Win32s is to convert between 32-bit and 16-bit memory addresses, an operation called thunking.
Many applications are designed to run on Win32s-enhanced versions of Windows for performance reasons. When you install such an application on a 16-bit Windows system, the installation procedure automatically installs the Win32s system if necessary.