The microcontroller on the Arduino board has EEPROM: memory whose values are kept when the board is turned off (like a tiny hard drive). This library enables you to read and write those bytes. The microcontrollers on the various Arduino boards have different amounts of EEPROM: 1024 bytes on the ATmega328, 512 bytes on the ATmega168 and ATmega8, 4 KB (4096 bytes) on the ATmega1280 and ATmega2560.