[spc476]

There are several types of ROM available. The type you would find in most 8-bit computers from Atari, Apple, Commodore, Radio Shack, etc. would be chips pre-programmed (aka a Mask Read-Only Memory) at a factory with the contents, such that each bit is set high or low and cannot be changed at all.

Then there's PROM (Programmable Read-Only Memory), which is "empty" (either all 0, or all 1, I don't know the full details) but can be programmed once and that's it. Then there's EPROM (Erasable Programmable Read-Only Memory), which can be erased (by exposing the actual chip to UV light), then reprogrammed. Then there is EEPROM (Electrically Erasable Programmable Read-Only Memory) which can be electrically erased and reprogrammed. For each type of ROM, once programmed, it good for years, if not forever.

The programming of a PROM, EPROM or EEPROM usually consists of applying a higher than normal voltage to the chip on certain pins and is usually done in a separate device. How these chips works internally (how the gates are arranged, erased, programmed, etc.) is not something I know (being into software). This is just stuff I picked up over the years.