WINE
WINE (Wine Is Not an Emulator) is a compatibility layer program that provides reverse engineered versions of Windows DLL files (that have no original Windows code for copyright reasons) to run Windows based programs in a Linux environment.
Unlike an emulator, which uses more processing power due to needing to retranslate the operating code from Windows to Linux, WINE runs Windows software at the same requirements as the native operating system since the DLL calls are being answered by equivalent code, which means that most Windows applications will run as if they were on Windows with little to no loss of performance or functionality.
WINE is not perfect software and is constantly in development, but it can allow the running of many Windows specific applications and games with partial to full functionality in a Linux environment, and can be incredibly invaluable for allowing the use of programs for which there is no Linux equivalent.