RetroArch can be many things to many persons. However, it is currently used by most as a modular multi-system game/emulator system. It suits that usecase well since it has been designed to be fast,
lightweight, and portable. It has features few other dedicated games / emulators have,
such as:

 

- Real-time rewinding
- Multi-pass shaders (up to 8-pass)
- Game aware shading
- Remote networked sound support
- A built-in GUI/OSD system

For a complete list of supported emulator cores and games visit the Libretro Wiki.