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, 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.
http://www.winehq.com/2. General Questions about Wine
2.1. What is Wine and what is it supposed to do?
Wine is a program which allows the operation of DOS and MS Windows programs (Windows 3.x and Win32 executables) on UNIX operating systems such as Linux. It consists of a program loader, which loads and executes a Windows binary, and a set of libraries that implements Windows API calls using their UNIX or X11 equivalents. The libraries may also be used for porting Win32 code into native UNIX executables, often without many changes in the source. Wine is free software, and its license (contained in the file LICENSE in each distribution) is the LGPL.
2.2. Does Wine emulate a full computer?
No, as the name says, Wine Is Not a (CPU) Emulator. Wine just provides the Windows API. This means that you will need an x86-compatible processor to run an x86 Windows application, for instance from Intel or AMD. The advantage is that, unlike solutions that rely on CPU emulation, Wine runs applications at full speed. Sometimes a program run under Wine will be slower than when run on a copy of Microsoft Windows, but this is more due to the fact that Microsoft has heavily optimized parts of their code, whereas mostly Wine is not well optimized (yet). Occasionally, an app may run faster under Wine than on Windows. Most apps run at roughly the same speed.
2.3. Are there any alternatives to Wine?
Yes, there are. You can use VMWare to run a Windows installation inside a virtual machine, or use Win4Lin to run a specially adapted Windows version on Linux. Both solutions cost money for both the software itself and a Windows license.
Note that, like Wine, they can only use the hardware platform that the target programs were originally compiled for (see below).
http://www.winehq.com/site/docs/wine-faq/index#GENERAL-QUESTIONS-ABOUT-WINE