Summary - Link tracking

Delivered	Total emails sent minus bounces: the number of emails that were successfully delivered.
Unique clicks	The number of recipients that clicked on any links. The same recipient may have clicked on multiple links. This is the number of unique readers that clicked. Therefore, it's a subset of READERS or UNIQUE OPENS.
Clicking readers without images	Recipients that clicked on at least one link (which is why we know they opened the message), but did not download any images.
Total clicks	The total number of clicks. The same recipient may click on multiple links, which means that CLICKS is always larger than UNIQUE CLICKS.
CTOR (Click to Open Rate)	Unique clicks divided by unique opens. It's an important indicator of the quality of the message: it indicates how many of those who opened a message also clicked on one of the links it contained.

WL User Guide