Read-only memory (ROM) is a type of storage used in computers and other electronic devices. The earliest ROMs were manufactured with the required data stored permanently on them and could never be updated.
Later versions, such as Erasable Programmable Read-Only Memory (EPROM) and Electrically Erasable Programmable Read-Only memory (EEPROM or Flash ROM) are are often referred to simply as ROM, despite the fact that they can be erased and re-programmed many times. Writing to this type of memory takes much longer than writing to RAM and uses different techniques.
These types of ROMs are often used for the storage of program code (firmware) and non-volatile data.
You can get more information about ROM from Wikipedia.