An Introduction to Emulating

Yes, it’s quite easily possible to play video games on your home computer. This is possible with programs called video game emulators.
A video game console emulator is a type of emulator that allows a personal computer or (in some rare cases, other video game consoles) to emulate a video game console’s hardware and behavior and play games for that platform. Emulators are most often used to play older video games on personal computers, but they are also used to play games translated into other languages or to modify (or hack) existing games. More often than not, emulators offer additional features above and beyond that of the original console, such as multi-controller compatibility (such as PSX controllers being used with N64 games and vice versa), timescale control, higher framerates, higher resolutions, unlocking of gameplay features, memory modifications (like GameShark), and one-click cheat codes. Emulators are also a useful tool in the development process of homebrew demos and the creation of new games for older or discontinued consoles.

Code and data of a game are typically supplied to the emulator by means of a ROM file (a copy of the data contained on a game cartridge) or an ISO disc image (for systems that use optical media).

Is this legal?
Some emulators, in adition to the ROM or ISO file require a BIOS file. Under United States law, obtaining a dumped copy of the original machine’s BIOS is legal under the ruling Lewis Galoob Toys, Inc. v. Nintendo of America, Inc., 964 F.2d 965 (9th Cir. 1992) as fair use as long as the user obtained a legally purchased copy of the machine. However, several emulators for platforms such as Game Boy Advance are capable of running without a BIOS file, using high-level emulation to simulate BIOS subroutines at a slight cost in emulation accuracy.

Most game titles retain their copyright even with the original system and games being many years past discontinuation and increasing rarity, so many resort to the obtaining of these games for free on ROM repositories and peer-to-peer networks rather than purchasing and ripping the ROM from the game (although, this is popular among those who already own the games). This is generaly considered piracy and a violation of copyright.
It s worth noting, a few game publishers have made legal ROMS avalable. Nintendo’s Virtual Console is one such case.