The original Nintendo joypad control system is emulated on screen, as shown in the screenshot below. The positioning and size on the controller can be adjusted through the app's settings. Being touch screen based it's not surprising that the on-screen controls are not as good as the original hardware, but they are more than sufficient for playing most games, and it is clear some thought has gone into to getting the placement right.
Emulators like Snes8x are somewhat controversial, with some question marks over their legality. In general the emulators themselves are legal, though this does vary from country to country, but playing commericial ROMS (i.e. games copied from original catridges is not). However, using public domain "homebrewed" ROMs, of which there are many for the SNES, is generally regarded as acceptable.
Windows Phone Store description:
Snes8x is a Super Nintendo emulator for Windows Phone 8. The code is based on Snes9x, a free and portable SNES emulator. The emulator contains one game that is freely available.
Snes8x, which is only available for Windows Phone 8, can be downloaded from the Windows Phone Store.