Retropie is an excellent addition to the software already available to the Raspberry Pi. It comes with a handful of emulators out of the box but is capable to run many more. The way to use them is a bit convoluted. First you execute the retropie cfg script and install an emulator you want from there e.g. snes, PS1 etc.

Now even though the emulator is installed, it won’t show up in emulation station. For that you have to put a ROM with the correct extension in the emulator folder. Then you can select the emulated system from emulation station.