Both the Neo 1973 and the Neo FreeRunner have NAND Flash.
The Neo FreeRunner also has NOR flash.
The FreeRunner keeps a boot loader in the NOR memory. This means that you can completely wipe the NAND memory (intentionally or otherwise!) and the FreeRunner can still be recovered by booting from NOR flash and loading new images into NAND via a simple USB connection (See Flashing the Neo FreeRunner).
For an article comparing the two technologies, see this page from Linux Devices: NAND vs. NOR flash -- Tradeoffs and Strategies
In short, NOR flash acts more like non-volatile RAM; programs can execute directly from NOR, and NAND flash acts more like a disk drive- all read/write operations are done in blocks not bytes and programs are first copied into RAM and then they are executed from RAM.
See also this Wikipedia entry: Flash memory