What does 'sync' exactly mean when using iTunes? I have been confused about using "sync" in iTunes. Let us say I have 


A laptop, and
An iphone.


When I connect the iPhone with the laptop, iTunes pops up inviting me to sync



By "sync", does it mean to


overwrite the content of the laptop with that in the iPhone, 
overwrite the content of the iPhone with that in the laptop, 
or something in between?
Synchronization is the process of ensuring that your iTunes and iPad/iPhone contain the same up-to-date files. If you add, change, or delete a file from one location, the synchronization process will add, change, or delete the same file at the other location.

For example, if you had a few songs on your computer, and a few on your phone, and the ones on your phone were not on the computer, and the ones on your computer were not on the phone, then after a "sync" operation, the contents of your computer AND the phone is the combination of contents of both devices. 

In short,  the content on device 1 now exists on device 2 alongside device 2's original content, and the content of device 2 now exists on device 1 alongside device 1's original content.