Definition
NAND flash is the semiconductor part inside USB drives and SSDs that holds your photos and files. It is packed with tiny rooms that can trap electrically charged particles. Even when the power is cut, the particles stay locked in those rooms, so your data doesn't disappear.
Why your presentation is still there after you unplug the USB drive
You save your presentation on a USB drive, pull it out of your computer, and toss it in your bag. A few days later you plug it into a school computer, and the file is right there. Not a drop of electricity reached that little stick the whole time.
It helps to picture the inside as a huge grid of tiny buckets. An empty bucket means 1. A bucket filled with water means 0. That looks backward, but it's the rule NAND follows. Every letter and photo your computer handles is really just a long string of these 1s and 0s.
Each bucket is called a cell. The "water" here is a tiny charged particle. The walls around the cell are so tightly built that the particles can't escape.
So even if you pull the plug, the water stays in the bucket. Keeping its contents without any power is the biggest feature of this part.
Filling and emptying buckets follows rules
To fill a bucket, you have to push particles through those tight walls. So when data is written, a stronger electric charge is applied than usual. Reading is much easier. You just peek to see whether there's water inside, which takes far less energy.
Here's the catch: every forced push wears the walls down a little. If you keep filling and emptying the same bucket, that one starts leaking first. So the controller chip inside rotates which spots get used and spreads the wear around.
Emptying has rules too. You can't empty a single bucket. You have to dump a whole block, a group of many buckets, all at once. That's why cleanup takes time even after you delete a file.
The controller chip handles all the housekeeping: picking buckets, emptying them, and moving data around. You just drag and drop your files and never have to think about what's happening inside.
A little more precisely
There's a way to use buckets even more efficiently. Instead of checking only whether there's water or not, you read the water level in several steps. Each bucket then holds more information, so more data fits in the same space.
But there's a price. The more finely you split the levels, the easier it is to misread a value when just a little water leaks out, and the fewer times the cell can be rewritten. The products on sale today are the result of balancing big capacity against long-lasting durability.
Finally, it's easy to mix this up with RAM inside your computer. RAM is a set of buckets that lose all their water when the power goes out, so it's a spot for holding the work you're doing right now. NAND flash is made of buckets that keep their water, so it handles long-term storage.
Common misconceptions
Data stored on NAND flash stays intact forever as long as the power is off.
The trapped particles slowly leak out over time. How long data lasts depends on storage temperature and how worn the chip is. The international standard (JEDEC) requires a consumer SSD that has been used up to its rated lifespan to keep its data for one year without power at 30°C (86°F).
When you delete a file, its buckets are emptied right away.
Emptying only works one block at a time, so the drive first marks the file as deleted and dumps the water later, all at once. That's why the old contents can stay inside the chip for a while even right after you delete them.
Where you meet it
Even if a game console suddenly loses power, your saved progress is still there.
You can switch off a smartphone for days, turn it back on, and every photo you took is still inside.
In one sentence
NAND flash is a semiconductor part packed with tiny rooms that keep particles locked in even without power, so your photos and files can be stored for a long time.