ROM is simply Read Only Memory.
Unlike RAM ROMs cannot be written to. ROMs are also typically slower then RAM.
ROMs come in several flavors:
PROM's can be programed ONCE then read many times. If there is any mistakes in the PROM code, the entire chip must be disgarded. While PROMs can be read in a normal board, they need to be programmed in a special device.
EPROMs can be erased after they have been programmed. Normally there is a small window in the chip in which exposure to UV light will erase the EPROMs programming. EPROM's need to be programmed in a special programmer.
EEPROMS can be erased electronically, and reprogrammed. While similar to Flash Memory, however they need to be erased & programmed in a special programmer.