This is a simplifed tutorial for Turing Students wanting to get started in RetroPie with a Raspberry Pi 2 or 3. I'm making the assumption that you have a MacBook with an SD card reader.
RetroPie is a combination of many emulation projects including EmulationStation and RetroArch. These projects are bundled into one system which is built on top of the Raspbian OS. EmulationStation essentially acts as the front end for a bunch of independently designed emulators.