Wine is an Open Source implementation of the Windows API on top of X and Unix.

Think of Wine as a Windows compatibility layer. Wine does not require Microsoft Windows, as it is a completely alternative implementation consisting of 100% Microsoft-free code, but it can optionally use native system DLLs if they are available. Wine provides both a development toolkit (Winelib) for porting Windows sources to Unix and a program loader, allowing many unmodified Windows binaries to run on x86-based Unixes, including Linux, FreeBSD, and Solaris.