NAND flash is a type of flash memory that is inexpensive, and comes with a certain proportion of bad blocks, and only allows block access. There is one 64 MB (nominal) chip on the Neo1973, while there is a 256 MB chip in the 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)
See Partitions to know how that memory is allocated for paritions to run the system.
See also the Wikipedia entry on Flash memory.