What is an emulator?
As every gamer knows, each game is designed especially for a specific device or gaming console. A game developed for one console will not run in any other device. Developers often release games across multiple platforms that cover almost every gaming devices there are. But some games, specially the older ones, are device specific. So you can not run every game on the device you own.
Unless you use an emulator.
What is emulation? Let me clear it with an example. Suppose you want to play mario kart games on your PC. But the games of this series is released on GameCube. So naturally, you will not be able to play it using your PC. However, if you had an emulator and a mario kart double dash rom, you could have run it easily. That is what an emulator does. It helps a player to run a game in a device, for which the said game was not released.
However, there is an ethical dilemma regarding it. Games that have a PC release is never truly out of market. You can find them if you look for them. But games that were released on consoles has a limited shelf-life. If you just look at the cost of playing an old game by using console, you would find the price to be monumental. But using emulator to a run a game that is still on the circulation would not be legally and ethically correct. So emulators are great tools that need judicious use.