EEPROM stands for Electrically Erasable Programmable Read Only Memory, and it is the name of a tiny chip which contains data code. This data can be erased, rewritten, or both, byte by byte, by an electrical charge. It is not possible to simply erase some EEPROM data while leaving the rest of it.
In order to rewrite any part of EEPROM data, it is necessary to erase the whole chip and then begin rewriting the data. EEPROM differs from RAM, or Random Access Memory, because Random Access Memory loses information every time a computer is shut down and turned off. EEPROM, however, retains all of its data and is not dependent on a source of power. Because of this, EEPROM has become the most popular means used by BIOS chips in order to save a system's settings without fear of losing any data.
The way that this works is that, when a computer powers up, the BIOS, or Basic Input/Output System, chip starts up a program which is meant to hold settings and allow the computer to recognize any and all hardware. With EEPROM, BIOS uses its chip to save the information, thereby eliminating the chances of a computer losing the memory of its proper settings. The interface between BIOS chips and EEPROM have finally made it possible to replace the BIOS chip if ever it fails, when in the past people in this predicament were required to buy an entirely new motherboard. EEPROM does have its disadvantages, such as the fact that works more slowly than RAM and thus does not do well programs and applications that have extensive and stringent requirements, such as digital cameras and flash cards.
However, a new, hybrid form of EEPROM, defined as flash memory, has been created. With flash memory, data can be rewritten in a selective way and it can be erased and rewritten in larger blocks than a single byte at a time.
Al Munroe writes on topics such as EEPROM, Flash Memory and BIOS for The Tech FAQ.