NAND

This refers to the NAND flash chip used in the Apple TV, iPad, iPhone, and iPod touch. This is where all the storage capacity comes from. The capacity in each chip can range from as little as 4 GB to as many as 128 GB. In iOS, it is split into two partitions: the system partition and the user partition. The system partition contains the iOS operating system and the native Apple applications, and is mounted as read-only. The user partition contains all user data, including installed applications, music, and movies, and is mounted as read/write. Jailbreaking modifies the file /etc/fstab to enable read and write access on both partitions.

Extras
NAND partition layout in modern devices, courtesy CPICH, published in Twitter (NAND partition layout)...