RAM cells need constant refreshing, however NAND FLASH cells can retain data for years together. What makes this possible ?
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.