Emulation is, generally, using a software to provide a different execution environment or architecture. Emulation mimics the externally observable behavior to match the target. The internal state of the emulation mechanism does not necessarily reflect the internal state of the target which it is emulating. Emulators emulate hardware without relying on the CPU being able to run the code directly. So it is more like a translation of one set of instructions into another.

For example, a PlayStation 2 emulator on a Windows machine enables you to play PS2 titles on your PC; an Android emulator lets you emulate an Android device on your PC or Mac.