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Non-volatile memory (NVM) or non-volatile storage is a type of computer memory that can retrieve stored information even after having been power cycled. Non-volatile memory retains it states even if the device is powered off. In other words, what's in non-volatile memory survives a device reboot.

Basically there are two types of non-volatile memory.
The first one is ROM and the second one is Flash-RAM.

Most of you know this memory type by its PC name, ROM. ROM stands for Read-Only Memory and it is usually based on a flash memory.
What does flash memory mean? Simple. That means that we can re-program this memory to hold new data. So what data is stored in the Flash ROM? It usually stores the OS and some other application related parts that we do not want to be erased.

On smartphones, some applications will see/show the ROM as the Z-drive. It can be viewed/read, but not written to. On newer devices based on Symbian 9.1/S60 3rd Edition, access to specific directories (or "folders", if you prefer that term) is also protected from unprivileged applications.

(Note: There are also two different Flash ROM technologies in use these days; "NOR Flash" and "NAND Flash". A device can have one or the other, or both kinds. Without going into detail, the main thing to remember about these is that programs on NOR Flash can be run/executed without first loading the executable file to RAM. Programs on NAND Flash must first be loaded into RAM before it runs. NAND Flash is cheaper, but then requires the device to have more RAM to accommodate that.)

The second kind of non-volatile memory, can be called "Flash RAM" as it is still based on the same Flash memory technology, but it is also writable (hence "RAM", as opposed to "ROM). This kind of memory is characterized as "user [storage] memory", or sometimes "phone memory". This is visible as the C-drive.

The C-drive is initialized with files/data that the operating system needs for various purposes (anything that needs a writable storage location that survives powering off the device). This is also the memory where by default things such as contacts, messages or photos are saved to "phone memory". Same goes for software applications (games or whatever) and their data files, when they are also installed to "phone memory".