Sony eReader: Difference between "Library" and "Reader" tabs?

I recently got one of these but the manual and help option is rather vague.

In the main menu of the Reader Library on my computer, one of the drop-down tabs reads "Library" while the other reads "Reader", the latter being where I drop things in to sync them to the eReader.

But both tabs have similar options, and the only real difference I can tell is that some of the things I sync don't appear under "Library" even though they appear under "Reader". What's the difference between the two?
EDIT: I meant Kindle.
 
Back
Top