Those devices are writeable onboard like e.g. RAM but keep their 
contents even without being powered.
Therefore these devices are also often called 'non-volatile' memory 
devices, or NVRAM.
MUses are the storage of e.g. MAC Adresses on network cards (you'll find 
one on every network card), parameters or data which are unique for a 
unit but need to be stored permanently like calibration data.

Ario K. wrote:
> serial and parallel flash memories ?

Both are nonvolatile memories. Serial devices must be accessed with a 
serial protocol, you cannot map it into the address space of a cpu. 
Parallel devices can be used like conventional memory at least when 
reading, e.g. they can be used as program memory.

Georg