It used to be that if you wanted to play a video game that wasn't made for the computer, you would have to go out and purchase the game and the system to play it on. Or, you'd have to take a trip to the local arcade and shell out some hard-earned quarters for the privilege. Today, gamers have another option in the form of emulators -- programs that allow you to play games from other consoles on your computer.
Emulators for older game systems require that you download or otherwise acquire a ROM image of the game you wish to play, while emulators for newer CD- or DVD-based systems may allow you to play your games straight from the original game disc. You will also need some kind of controller device such as a keyboard or joystick, although some companies do manufacture USB-driven reproductions of original console controller pads that can be used with your PC's hardware.
How They Work
Emulators simulate -- or "emulate" -- the hardware of whatever system they are based on. An NES emulator, for example, will mimic the workings of an old NES game system, with a few key differences. Because it is on a PC, it obviously cannot play the old cartridge-based games. These games must be downloaded from their original game cartridge onto a PC in ROM form by using special cables. Once on the hard drive as a ROM file, the games can be opened and read by the emulator and displayed on your PC screen.
Emulators exist for almost every old video gaming system in the world. Many groups are even working on emulators for current-generation gaming systems. As of June 2011, working emulators exist for every console or arcade cabinet (via the use of the MAME emulator) up through the PlayStation 2/GameCube/Xbox era.
The legality of emulators is something of a gray area. Emulator users often maintain that as long as you own the game you are emulating, there is no breach of law. Video game companies, however, often contend that even the existence of an emulator is an infringement on intellectual property laws and should be illegal. Because there are no court cases dealing specifically with the legality of emulators to set precedent, the legality of emulators -- especially those for older systems no longer officially supported by their parent companies -- remains uncertain.