The Famicom Disk System was an add on for the Famicom in 1986 that used a non-standard 3" disk. It allowed games to be put on floppy disks instead of on cartridges. This was advantageous because disks were rewritable media, but disadvantageous are lower quality sound and graphics.
A special feature in the add-on is that you can save your game progress directly in the disk rather that use a password. This is pretty much useful for RPG games.