What Is An Emulation?

I've never heard of or seen a "gaming emulation" that I'm aware of and I need some clarification on exactly what one is. I think it must be similar to a modification or cheat software or even a debugging program. Just stop me if I guess the correct answer... I also certainly hope that all these "emulations" are perfectly legal in the regions they're used in.

Emulators allow you to use another system on your PC. In other words, it allows you to run Nintendo, Super Nintendo, Nintendo 64, Sega Genesis, Sony PlayStation and many more games on your PC. The difference is that there are big improvenents compared to the original system, like texture quality, high resolution, better sound, various support, cheats, quick saves, etc.