NAND and NOR Flash

NOR flash used as main memory
NAND flash used as secondary memroy.

In embeded device:
used as main memory: ROM, EPROM, NOR flash, SRAM, DRAM
used as secondary memory: NAND flash, SD, DOC(disk on chip).

NOR and NAND flash differ in two important ways:

• the connections of the individual memory cells are different
• the interface provided for reading and writing the memory is different (NOR allows random-access for reading, NAND allows only page access)

http://en.wikipedia.org/wiki/Flash_memory#Distinction_between_NOR_and_NAND_flash