RAM cells need constant refreshing, however NAND FLASH cells can retain data for years together. What makes this possible ?
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What makes NAND memories non-volatile is that the logic state is retained in a physical state that doesn't depend on the supply voltage.
For EEPROM memories in general (including Flash), this is a charge trapped in a floating gate.
As clabacchio pointed out, NAND flash memories can stores information in an array of memory cells made from Floating-gate MOSFET. Also some newer flash memory(included NAND memories) has a new technology. it's Multi-level cell.
I think these links will be very helpful and useful:
Note: I know that they have spoken about Flash memory in these links but the technology that they have spoken about it is NAND type.
Also note that data retention is usually guaranteed for a period, e.g. 10 years. Also note that bit flipping can occur. SD cards for example usually have ECC built in to correct this, to some extent.