Both FLASH and EEPROM are electrically erasable memories and use similar principles to achieve this. Erasing and writing these memories requires a 'high' voltage of around 12V. Some (older) types need an external voltage, but most modern types have an inbuilt converter and need only a single 5V or 3.3V supply voltage.

EEPROM has the ability to erase one byte at a time. This feature makes it very useful for long-term data storage that has to be changed from time to time like calibration data for example. The drawback is higher cost, due to greater complexity. Erasing and programming of a byte takes a relatively long time; typically several milliseconds.

FLASH can only be erased in blocks, varying from 256 bytes to several K or even the whole array. This makes FLASH memory simpler, thus cheaper, than EEPROM. This simplicity also means that FLASH arrays can be made larger, currently up to 512 MBit (it might even be 1GBit). Erasing a FLASH block takes only about as long as one byte EEPROM and most devices have write cycles of around 100 µs to several ms. Read cycle time is comparable to 'standard' EPROMs; 70 - 120 ns with 55 ns available.

Hope this makes things a bit clearer.

Rob.