ROM stands for Read Only Memory, a type of computer memory on which data has been prerecorded. Once data has been written onto a ROM chip, it cannot be removed and can only be read.
Unlike main memory (RAM), ROM retains its contents, even when the computer is turned off. ROM is referred to as being 'Non Volatile', whereas RAM is called 'Volatile'.
Most personal computers contain a small amount of ROM, that stores critical programs, such as the program that boots the computer, and runs the POST. In addition, ROMs are used extensively in calculators, and peripheral devices, such as laser printers, whose fonts are often stored in ROMs.
A variation of a ROM is a PROM (Programmable Read Only Memory). PROMs are manufactured as blank chips on which data can be written with a special device called a PROM programmer.