Definition
NAND flash is a non-volatile storage technology: it keeps its data with the power off. Its memory cells are wired in series in a pattern resembling a NAND gate, which gives the technology its name, and it underlies most solid-state storage.
How it works
NAND represents bits as electrical charge trapped in floating-gate or charge-trap cells. Reads and writes happen at page granularity while erasure happens on much larger blocks, and a block must be erased before it is rewritten. Cells wear out over erase cycles, so controllers apply wear leveling and error-correcting codes (ECC); to raise density, the industry stores multiple bits per cell in SLC, MLC, TLC, and QLC arrangements.
Usage
SSDs, phone storage, memory cards, and USB drives are all built on NAND media; data centers likewise run databases and virtualization workloads on NAND arrays.
Related and confusable terms
An SSD is a finished device built on NAND media plus a controller and an interface — the two live at different layers. RAID and NAS are ways of organizing multiple storage devices for redundancy and sharing, unrelated to the chip-level technology itself.