RetroArch may be the single greatest contribution to classic gaming emulation since the dawn of Nesticle: a multi-console emulator frontend spanning the history of videogames from the Atari 2600 through Playstation eras. Libretro is the companion to RetroArch that contains all the emulator cores.
Installing RetroArch and Libretro in Windows or OSX is a fairly simple process of downloading the RA binary and the Libretro cores, but in Linux, it takes a little more effort.
First, add the Hunter Kaller repository to Ubuntu and update:
sudo add-apt-repository ppa:hunter-kaller/ppa
sudo apt-get update
Install RetroArch and Libretro with a couple of terminal commands:
sudo apt-get install retroarch
sudo apt-get install libretro*
When you run RetroArch, the Libretro cores will be located in /usr/lib/libretro/