Read-Only Memory (ROM) is a non-volatile memory; memory is not lost when the computer is switched off.
ROM can be read from, however, it can not be written to, hence the term 'read-only'.
This makes ROM ideal for storing instructions and data the computer needs to operate. These instructions and data are usually programmed by the computer's manufacturer and cannot be overwritten
The BIOS (Basic Input Output System) a program that is stored in ROM. The BIOS runs as soon as the computer is switched on. It checks that the hardware is working correctly, it then runs a second program called the boot-up or bootstrap program that loads the computer's operating system (OS) from the hard drive and into the RAM. The BIOS is always needed so therefor it is stored in ROM.
TIP - ROM can be read from but it cannot be written on.