Programmable Read Only Memory (abbreviated PROM) is a memory chip on which data can be written only once. Once data has been written onto a PROM chip, it remains there indefinitely. Unlike RAM, PROMs retain their contents when the computer is turned off.
The difference between a PROM and a ROM (Read Only Memory) is that a PROM chip is manufactured as blank memory, whereas a ROM is physically programmed during the manufacturing process. To write data to a PROM chip, you need a special device called a PROM programmer or PROM burner. The process of programming a PROM is sometimes called burning.
EPROM (Erasable Programmable Read Only Memory) is a special type of PROM that can be erased by exposing it to Ultraviolet Light. Once it is erased, it can then be reprogrammed. An EEPROM is similar to a PROM, but requires only an electrical current to be passed through the chip for it to be erased.