This is the software publisher's description.
Wine is an Open Source implementation of the Windows API on top of X and
Unix.
Think of Wine as a compatibility layer for running Windows programs. Wine
does not require Microsoft Windows, as it is a completely free alternative
implementation of the Windows API consisting of 100% non-Microsoft code, however
Wine can optionally 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, Mac OS X, and Solaris.
The Wine project started in 1993 as a way to support running Windows 3.1
programs on Linux. Bob Amstadt was the original coordinator, but turned it over
fairly early on to Alexandre Julliard, who has run it ever since. Over the
years, ports for other Unixes have been added, along with support for Win32 as
Win32 applications became popular.
Wine is still under development, and it is not yet suitable for general use.
Nevertheless, many people find it useful in running a growing number of Windows
programs. Please see the Application Database for success and failure reports
for hundreds of Windows programs, as well as the Bug Tracking Database for a
list of known issues, and the Status page for a global view on Wine's
implementation progress.