AVR microcontrollers have some amount of EEPROM memory on-chip. For instance, Atmega328 has 1K of byte-addressable EEPROM. EEPROM memory can be used to store and read variables during program execution and is nonvolatile. It means that it retains values when the power supply is off. EEPROM memory comes in handy when we need to store calibration values, remember program state before powering off (or power failure) or store constants in EEPROM memory when you are short of program memory space, especially when using smaller AVRs. Think of a simple security system – EEPROM is the ideal place to store lock combinations, code sequences, and passwords. AVR Datasheets claim that EEPROM can withhold at least 100000 writes/erase cycles.