An emulator is a complete re-implementation of a particular device or platform. The emulator works exactly like the real device. For example, an NES emulator implements the CPU, sound chip, video output, controller signals, and so on. The unmodified code from an NES cast can be downloaded and then the resulting image can be loaded into the emulator and played.
A simulator is a partial implementation of a device / platform, but only for its own purposes.
