@Downup
What my dear friend oldchocobo meant to say was:
Emulationstation is the front end of all of your emulators and as such can “support” displaying any system that is placed in the es_systems.cfg file.
Their site gives a pretty good explanation of how the es_systems.cfg file works:
http://www.emulationstation.org/gettingstarted.html
Emulationstation in it’s simplest form is a skin for your emulators. Lots of people use it on their PCs as well and as their PC’s have more powerful hardware they’ll have more systems such as the PS2 and Wii.
But the Raspberry Pi’s hardware isn’t as powerful as a PC so there are fewer emulators supported on the Pi- as a result you’ll have less systems configured in emulationstation.
Because of how seamlessly the RetroPie project and Emulationstation work together it can be easy to see the two as interchangeable.