ROM

ROM is Read Only Memory it cannot be changed and can only be read. This is useful for shipping firmware or operating systems on because the code cannot be changed, this reduces any issue of code not working. It is often used for firmware and OS' because they aren't going to be changed regularly, therefore they don't need to be writeable. ROM is not volatile and retains its memory when power is lost, compared to RAM that is volatile.

PROM is Programmable Read Only Memory; this is how ROM is written, the information is written and then it cannot be changed again because it has become Read Only.

Most computers contain a small amount of ROM, such as the bios and the program that boots the computer.