NAND vs NOR?

When do I use NAND, when NOR? Why? For example I look in NAND folder and I see this README: https://s3.us-east-2.amazonaws.com/download.gl-inet.com/firmware/ar300m/nand/README.txt

It does tell me NAND != NOR. But not explaination what is NAND, what is NOR, why I should use one, why the other one? There is no explanation of firmwares and this makes so many problems to flash new.

The 300M has 2 different memory chips. A 128MB NAND memory chip and a 16MB NOR memory chip.

If the firmware on the 128MB NAND chip fails to boot 3 times in a row, the firmware on the NOR memory is booted instead.

Thank you but yes I know these informations. It does not tell me new things.

My question is what is the difference and which one do I want to install please?

(Also I am really wanting someone to check that firmware can be installed because I try and it will not boot to NAND also not boot to NOR: Firmware for 300m16 Not working )

Also it is a little funny if NAND is the “normal” one, but it exists only in specific /nand directory. Also why is not new /tor firmware for NAND but does exists for NOR? Seems very strange.

Also why exist .img and also .tar? GL.iNet download center