Ok, so KB = kilobyte = 1024 bytes (not 1000!) and therefore 64Kb = 65,536 bytes (64 x 1024). Flash memory is divided up into chunks of various sizes - and these vary (I think!) depending on the storage capacity of the chip. So Erase 64Kb block does what it says, it erases 64Kb of memory. You usually have to tell the chip which 64Kb block you want to erase.
I'm looking at a datasheet now for a flash device and it says it has 4Kb sectors, so a sector holds 4096 bytes. Therefore Erase Sector on this device will erase 4Kb of memory.