NES.app uses the popular InfoNES emulation core to mimic the 6502 processor in a Nintendo Entertainment System, allowing you to play ROM dumps of games designed for the NES console. ROM dumps are files containing the dumped instruction code from a physical cartridge.
NES.app is based on the same code as iPhoneNES, but because their authors couldn't keep up with all of the NES.app changes, NES.app has its own site and svn repository.