An EEPROM keeps the floating-gate storage of the EPROM but erases electrically. No ultraviolet lamp, no removing the chip: the circuit itself can erase and rewrite it.
Its defining feature is fine-grained rewriting: individual bytes can be changed without disturbing their neighbors. That makes it ideal for small amounts of data that change now and then and must survive power-off:
- calibration values
- user settings in appliances and car electronics
- serial numbers and configuration
Limits:
- Slow writes compared with RAM: milliseconds per byte rather than nanoseconds.
- Wear: each cell survives a limited number of erase cycles, typically hundreds of thousands to a million.
- Larger cells than flash, so less capacity for the money.
Flash is its descendant. It gives up byte-level erasing (it erases in blocks) to gain density and cost. Both are non-volatile.