An EEPROM (also called an E2PROM) or Electrically-Erasable Programmable Read-Only Memory, is a non-volatile storage chip used in computers and other devices (such as USB flash drives, in its flash memory version).
Operation
The EEPROM operates in three phases: OP-Code Phase, Address Phase and Data Phase. Each EEPROM device has its own set of OP-Code instructions to map to different functions. Some of the common operations on the EEPROM devices are Write Enable (WREN), Write Disable (WRDI), Read Status Register (RDSR), Write Status Register (WRSR), Read Data (READ), Write Data (WRITE). Other operations supported by some EEPROM devices are Program, Sector Erase and Chip Erase commands.

The OP-Code is usually the first 8-bits input to the serial input pin of the EEPROM device, followed by 16 to 24 bits of addressing depending on the depth of the device. The final phase of operation is the data phase. In case of WRITE, the controller needs to supply data till the chip select is low. In case of read, the controller needs to sample the data every serial clock edge till the chip select is low.

EPROM has two significant limitations - it has to be taken out of the circuit and put under an ultraviolet light source to erase it, and non-TTL voltages are generally required to program it.

EEPROM can be programmed and erased electrically.

The deference between EPROM and EEPROM lies in the thickness of the insulating layer - in an EPROM it is about 3 nanometres thick whereas in an EEPROM it is much thinner - typically around 1 nanometre.

EEPROMs are still limited in that repeated write and erase cycles eventually damage the thin insulating layer. Some early EEPROMs could only perform about 100 erase-write cycles.

EEPROM should not be confused with RAM (Random Access Memory). While RAM can be re-written many billions of times, EEPROMs can be written only a few hundred to a few thousand times. Also, RAM is generally a lot faster to write than EEPROM (typically a few nanoseconds as opposed to a few microseconds), and most types of RAM are volatile (they lose their contents when power is removed). EEPROM retains its data after powerdown.
Uses
Serial EEPROMs come in a range of capacities from a few bytes to over 128 kilobytes. They are typically used to store configuration parameters, and in modern computers they replace the hitherto CMOS nonvolatile BIOS memory. Unlike EPROMs, EEPROMs can be programmed and erased electrically. This is the main advantage EEPROM chips are commonly used by many Basic Input/Output systems (BIOS) chips to save system settings.

Flash memory is a later form of EEPROM. In the industry, there is a convention to reserve the term EEPROM to byte-wise writable memories compared to block-wise writable flash memories. EEPROM takes more die area than flash memory for the same capacity because each cell usually needs both a read, write and erase transistor where in flash memory the erase circuits are shared by blocks of many (often 512×8) cells.

Newer technologies such as FRAM and MRAM are slowly replacing EEPROMs in some applications.