ROM stands for Read Only Memory.
The ROM is non-volatile memory;it retains stored information even if the power is turned off (unlike RAM).This is basically used for storing programs and data that need not be altered one stored.
Processor uses ROM to store and execute programs instructions.However it is a type of prime memory and not a storage memory,therefore don't compare it with Hard Disks or CDs and DVDS etc
As the name itself suggests,the information can only be read not deleted,altered or even overwritten.This means that once a bit pattern is stored ,it is permanent or at least semi-permanent (if supports alteration).
The permanent group includes two types ,viz. ,ROM and PROM (Programmable ROM).
The semi-permanent also includes two types ,namely,EPROM (Erasable PROM)and EE-PROM (Electrically Erasable PROM).
Ashish