EEPROM stands for Electrically Erasable Programmable Read Only Memory, and is pronounced double-ee-prom or e-e-prom. It's a long name for a small chip that holds bits of data code that can be rewritten and erased by an electrical charge, one byte at a time. EEPROM data cannot be selectively rewritten; the entire chip must be erased and rewritten to update its contents.

While Random Access Memory (RAM) loses its data every time you power down your computer, EEPROM does not require a power source to maintain its data. For this reason EEPROM is commonly used by many BIOS chips to save system settings.

BIOS stands for Basic Input/Output System. When you turn on your computer the BIOS chip executes a program called CMOS (Complementary Metal Oxide Semiconductor) that holds settings, which enables the computer to recognize your hardware. You can enter the CMOS program during boot-up to modify BIOS settings. You might need to do this, for instance, when you get a new hard drive. After you modify the settings the BIOS will save the new copy of instructions to EEPROM.

With the advent of EEPROM, manufacturers could also update the BIOS program itself. In the past this wasn't possible, and an outdated BIOS chip meant having to replace the chip by getting a newer motherboard. A BIOS chip that is upgradeable using EEPROM capability is called a flash BIOS, because the EEPROM is updated using electrical charges or flashes.

EEPROM is slower than RAM, but is perfectly fine for applications such as storing saved BIOS settings. EEPROM would not be chosen for applications with dynamic read/write requirements, as in the case of a digital camera, memory stick, or flash card. For these purposes a newer hybrid form of EEPROM is used called flash memory. Flash memory differs from EEPROM in that its data can be selectively rewritten. It can also be erased and rewritten in entire blocks, rather then one byte at a time. This makes it much faster than EEPROM.

Newer flash BIOS chips may or may not use flash memory, rather than EEPROM. The BIOS is only called a flash BIOS because the memory it uses -- in both cases -- is reprogrammed by flashing the chip, either one byte at a time, or in blocks.