Photo Index File
When you use a Nikon Digital Camera you will find that there is a files saved to your media card with the extension NJB.
This file is used to index the photo´s on the card and is used when you view the photo´s on the LCD display that is usually found on cameras today.
When photo´s are deleted or you take photo´s then the njb file is updated automatically.
These files are usually not to be opened by the user, if this files is corrupted your camera may not work correctly.