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.

Binary Compatibility
• Loads Windows 9x/NT/2000/XP, Windows 3.x and DOS programs and libraries
• Win32 compatible memory layout, exception handling, threads and processes
• Designed for POSIX compatible operatings systems (eg. Linux and FreeBSD)
• "bug-for-bug" compatibility with Windows

Graphics
• X11-based graphics allows remote display to any X terminal
• X11, TrueType (.ttf/.ttc) and Windows Bitmap (.fon) Fonts
• DirectX support for games (limited Direct3D support)
• Support for OpenGL based games and applications
• Printing via PostScript driver or legacy native Win16 printer drivers
• Enhanced Metafile (EMF) and Windows Metafile (WMF) driver
• Desktop-in-a-box or mixable windows
• Windows MultiMedia (WinMM) layer support with builtin codecs

Allows Windows program to interface with:
• Sound devices via ALSA, OSS, ARTS, JACK, and libaudio etc
• Multi-lingual keyboards and CJK input method support via XIM
• Modems, serial devices
• Networks (TCP/IP and IPX)
• ASPI Scanners
• Windows Tablets via XInput (eg. Wacom)

Wine API
• Designed for source and binary compatibility with Win32 code
• Win32 API test suite to ensure compatibility
• Compilable on a wide range of C compilers
• Permits mixing of Win32 and POSIX code
• Permits mixing of ELF (.so) and PE (.dll/.exe) binaries in one address space
• Win32 compatible header files
• Automatically generated API documentation
• Resource compiler
• Message compiler
• IDL compiler
• Extensive Unicode support
• Internationalization -- Wine supports 16 languages
• Built-in debugger and configurable trace messages
• External memory checker support using Valgrind
• Sample programs

You can download this file at Wine's official website.