Read-only memory is a computer memory on which data has been pre recorded. Once data has been written onto a ROM chip, it cannot be removed and can only be read.
Unlike the main memory(RAM), ROM retains contents even when the computer is turned off. ROM is referred to as being non volatile whereas RAM is volatile.
Most personal computers contain a small amount of ROM that stores critical programs such as the programs that boots the computer.
In addition, ROM's are used extensively in calculators and peripheral devices such as laser printers. A variation of a ROM is the PROM, PROMs are manufactured as blank chips on which data can be written.