An 'opened' event is detected when an embedded 1x1 pixel transparent image is loaded by the email client. When a user views the email and chooses to view images this event is triggered. It is used as an indication of the number of people who have viewed the email. 

A 'click' event is much more deterministic and corresponds to the user clicking on a link embedded in the email. As those links are all coded, the user's click is tracked by our software and reported. In the context of when an email's objective is to drive traffic to the website this metric is the most useful and most accurate.