RetroArch is a frontend for the Libretro emulation API. It effectively functions as a multi-system emulator, although that is technically not correct. Its main strength is the number of systems it has emulation "cores" for and the number of platforms it can run on. RetroArch runs on everything: Windows, Mac, Linux, Android; you name it, and it will have a port for it.
The Libretro API allows developers to turn emulators into "libretro cores," which any compatible Libretro frontend can use. Among the available Libretro cores are ones based on Mednafen (as "Beetle") and MAME, which both are multi-system emulators by themselves. So it would be pointless to list all the supported systems here.