Install it:
$ sudo apt-get install retroarch
There are the emulators, RetroArch only is a front end.
$ sudo apt-get install libretro-nestopia
$ sudo apt-get install libretro-desmume
$ sudo apt-get install libretro-mgba
You must enable non-free repository.
$ sudo apt-get install libretro-snes9x
You must enable non-free repository.
$ sudo apt-get install libretro-mupen64plus
Genesis / Mega Drive, Sega/Mega CD, Master System, Game Gear & SG-1000
$ sudo apt-get install libretro-genesisplusgx
$ sudo apt-get install libretro-gambatte
There are the roms of games, RetroArch only is a front end.