Wine (originally an acronym for "Wine Is Not an Emulator") is a compatibility layer capable of running Windows applications on several POSIX-compliant operating systems, such as Linux, Mac OSX, & BSD. Instead of simulating internal Windows logic like a virtual machine or emulator, Wine translates Windows API calls into POSIX calls on-the-fly, eliminating the performance and memory penalties of other methods and allowing you to cleanly integrate Windows applications into your desktop.
To install the latest development version of Wine the package needs to be individually unmasked:
# echo "app-emulation/wine" >> /etc/portage/package.accept_keywords/wine
# emerge --ask app-emulation/wine
Setting up the environment
Create a 'windows installation' in the home path of a user (in this case /home/<user>/wine):
$ WINEPREFIX=~/wine wineboot