This article is from the WINE WINdows Emulator FAQ, by Dave Gardner firstname.lastname@example.org with numerous contributions by others.
Wine is a program which allows the operation of DOS and MS Windows
programs (Windows 3.x and Win32 executables) on UNIX. It consists of
a program loader, which loads and executes a Windows binary, and a
library that implements Windows API calls using their UNIX or X11
equivalents. The library may also be used for porting Win32 code into
native UNIX executables.
Wine is free software, and its license (contained in the file LICENSE in
each distribution) is BSD style. Basically, this means that you can do
anything with Wine except claim that you wrote it.