Memory chips are simply a collection of registers, each with its own address. Data, in the form of 0's and 1's, is stored in the registers.
ROM chips can be read from, but not written to. They are non volatile, which means that they retain their contents after power is removed. Most ROMs are programmed during manufacture of the chips.
Others, PROGRAMMABLE ROMS, PROMS, can have their contents programmed in after manufacture. The 2716 ROM shown above is an
This is an erasable prom, where if you make a mistake, you can erase the contents by shining ultra-violet light through a window in the chip.
Some chips are ELECTRICALLY ERASABLE and are known as EEPROMS.