Definition:
An emulator, in the most general sense, duplicates the functions of one system with a different system, so that the second system appears to behave like the first system.
Unlike a simulation, it does not attempt to precisely model the state of the device being emulated; it only attempts to reproduce its behavior.
The idea behind emulators is that by using an emulation software you can play video games on a system other than the one it was developed for originally.
Examples:
SNES Roms can be emulated to play on a PC. Xbox games can be emulated to play on an Xbox 360, making them backwards compatible.
