Wine is an Open Source implementation of the Windows API on top of X and Unix. It is a completely free alternative implementation of the Windows API consisting of 100% non-Microsoft code, although it can also use native Windows DLLs if they are available. Wine provides both a development toolkit for porting Windows source code to Unix as well as a program loader, allowing many unmodified Windows programs to run on x86-based Unixes, including Linux, FreeBSD, and Solaris.
Relation to ReactOS
ReactOS works with the WINE project to share as much programming effort as possible. ReactOS depends on Wine mainly for user mode DLLs. Where appropriate, patches to Wine are also submitted by the development team, and patch contributors are often directed to Wine if it is felt that the patches would benefit them.
However, due to architectural differences arising from Wine targetting the Linux platform, some of their DLLs may not be used on ReactOS without specific modifications. As an example, kernel32.dll and gdi32.dll have to be forked, since the Wine versions effectively redirect calls to the Linux kernel and X server respectively. More recently however a research effort has been made to bring the ReactOS architecture closer to Wine's in order that more of their code can be used without modifications. This is currently being done in the Arwinss branch.