RetroPie is a project based on Libretro, RetroArch, and EmulationStation. EmulationStation is an open source frontend that makes it easy to manage over 30 video game consoles and applications, and your associated game files. It includes a built-in joystick setup tool and can automatically download associated metadata for your files. This metadata includes items such as box art and game info, which gives your collection a polished finish.
Libretro is another open source project. It is designed to bring multiple emulators into one main package. Instead of having a separate application for each system, Libretro turns these emulators into a core, which hooks into the library. By having a common library for each emulator to hook into, a cohesive easy-to-use platform is created for the user. RetroArch is the main frontend for the Libretro library.
The RetroPie project combines the best of the emulation, gaming, and frontend systems into an easy-to-use platform for the Raspberry Pi.