In emulation, ROM refers to a ROM image taking by dumping the data from a game's ROM chips. On arcade boards this is done by removing each chip (they are usually socketed) and reading the information from them one at a time. For cartridge-based systems, hardware is usually available to read the data from the cartridge into a standardized data format. If no such hardware is available, the ROM chips can be read individually in the same fashion as an arcade game. The process is not limited to just game information. System software (such as a BIOS) can be dumped into a ROM or data from floppies used on an old computer system can be read in its raw format and then saved, to be later read by an emulator.
Dumping the data can be made difficult by encryption or compression of data. Some cartridge games included specialized chips that handled data compression and often there is little to no information available about how these chips operate. Later NeoGeo carts had the data on the ROMs stored in an encrypted form and decrypted before being being sent to the CPU.
ROMs can range in size from 2 KB for Atari 2600 games to over 80MB for newer arcade games or the NeoGeo carts.
Writers of emulators sometimes must make use of only the ROM data if no technical documentation is available for the hardware they are attempting to emulate.
See also: mask work, graphics pack