|Most classic games
are stored on game cartridges.
For example the Atari 2600, Nintendo
NES and Sega Genesis (among many
others) use game cartridges. The
games are stored on those cartridges
using ROM chips (Read Only Memory).
Some people have created special
devices which can read the data
from those ROM chips and which
can save that data on a PC in
a ROM file. In other words, a
ROM file contains all data from
a game cartridge. Of course for
systems which don't use game cartridges,
such as the Playstation, ROM files
don't exist. In case of the Playstation
you can put the Playstation CD
in your CD-ROM drive to play games
on a Playstation emulator.