NAND flash is a type of flash that is inexpensive, and comes with a certain proportion of bad blocks, and only allows block access. There is one 512 megabit (nominal) chip on the Neo1973. This will increase to two 512megabit chips in GTA02.
The capacity is nominal - more blocks are available early on, and fail over the use of the device, with the specified capacity guaranteed to be there at the specified number of writes. (100K in this case)