Roms are essentially an image which contains a copy of the data from a read-only memory chip, often found in older video game cartridges or on the boards of arcade games. The term is generally used in tandem with emulation which utilizes software to Decode the Firmware needed to run the Rom.
ROM images are also used when developing for embedded computers. Software which is being developed for embedded computers often write to ROM files for testing on a standard computer before it is written to a ROM chip for use in the embedded system. For the scope of this article, we will be discussing emulation.
ROM images in computer systems have in time been replaced with removable media such as CD and DVD and magnetic media such as hard drives and tapes. Recently, flash memory has made a surge as popular ways to store temporary data as well. With that, most talk of ROM images derive from the dumping of data from a chip embedded in a cartridge or computer board.
ROMs can be copied from their read-only chips using a dedicated device with a process known as dumping. For home video game cartridges, devices that can pull the ROM data are fairly common and easy to come by. Dumping a ROM from an arcade machine however usually takes individual setups for each machine and a large amount of experience to do correctly.
Some argue that ROM collecting is used in an effort to preserve aging technology, it is important for some companies to attempt to protect their assets on ROM images that are still considered market viable. For instance, the Nintendo DS is still cartridge based and dumps of these games can still be found circulating on the Internet. People who download these ROMs and do not have a legal copy of the game essentially hurt the market for said devices. To combat this, companies over the years have developed several copy protection schemes, similar to DRM that the music industry employs. Since its inception, there hasn't been a copy protection scheme that hasn't been later cracked by savvy ROM hackers.
ROMs can be collected widely through the Internet over several protocols including HTTP and BitTorrent. It is beyond the scope or desire of this article to inform you exactly how or where to obtain these ROMs. While ROMs themselves are not illegal, there are circumstances which could lead to trouble should you be caught with these images. In some countries, it is legal for the owner of a game to make a backup of the game in the interest of preserving the data. It has been said that digital data has an average shelf-life of 10 years, therefore preserving your purchase by dumping the ROM can be viable. In the US, it's been illegal since 1983 when Atari won a court case against making game backups. Additionally, some websites that offer ROMs for download claim that it is legal to download and keep a ROM for a period of 24 hours whether you own it or not, so long you understand it is your own responsibility to delete it after the time has expired. This claim, although widely believed, is completely false and no such law has ever existed.