ROM

Read Only Memory is physical memory that is not-volatitle, meaning, if there is not electricity, the memory is not lost. ROMs are used to put programs which start hardware compontents. and good example of this is the BIOS on your computer. these types of softwares are called firmware.

ROMs server their purpose, but have their limitations because once you burn the information into a ROM chip, it cannot be chaged, that is why they come up with new types of ROMs there are three

PROM

Programmable read-only memory - on this type of memory, you can write into just once, you cannot erase the data once you store it in a PROM

EPROM

Erasable Programmable read-only memory - on this type of memory you can erase the whole data and rewrite with a new one. meaning, you have to erase the everything and put new data, but you cannot keep any of the old data once you overide it with the new one - BIOS and CMOS use EPROM

EEPROM

Electronically Erasable Programmable read-only memory - on this type of ROM you can edit/modify the data and still keep your data.

as you can see from the image above, all these chips look alike, the difference in the internal circuitry. some examples of these chips on computers are the computer CMOS and BIOS. also the difference in these chips are the prices. EEPROMs are more expensive then ROM chips