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.
More information can be read in the articles Why Wine is so important, and Debunking Wine Myths. If you are wondering how well a particular application works in Wine, please examine the Applications Database. For installation instructions and step-by-step help with running Wine, take a look at the User Guide.
Wine is free software. The licensing terms are the GNU Lesser General Public License version 2.1.