A PROM is a memory chip on which data can be written only once. Once a program has been written on to a PROM, it remains there forever. Unlike the main memory, PROMs retain their contents when the computer is turned off.
The difference between a PROM and a ROM that is that a PROM is manufactured as blank memory, whereas a POM is programmed during the manufacturing process. To write data onto a PROM chip you need a special device called a PROM programmer or a PROM burner. The process of programming a PROM is sometimes called burning the PROM.