The memory which is programmed at the time is manufactured.
The ROM is programmed at the time is manufactured.
PROMs are used in digital electronic devices to store permanent data, usually low level programs such as firmware (microcode). The key difference from a standard ROM is that the data is written into a ROM during manufacture, while with a PROM the data is programmed into them after manufacture.
Read-only memory (ROM) is similar in design to static or dynamic RAM circuits, except that the “latching” mechanism is made for one-time (or limited) operation. The simplest type of ROM is that which uses tiny “fuses” which can be selectively blown or left alone to represent the two binary states.