What do "NAND", "ROM" and "to flash" mean?

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What is the difference between: Rooting, Jailbreak, ROM, Mod, etc.?

I'd like to know the meaning of the following terms:

• What does NAND mean?


• What is a ROM? Is it a custom OS?



• What does "to flash" refer to (as in, to flash a ROM)? Does it mean to copy?