When a hotlink is included in an email, a clickthrough occurs when a recipient clicks on the link. clickthrough tracking refers to the data collected about each clickthrough link, such as how many people clicked it, how many clicks resulted in desired actions such as sales, forwards or subscriptions.

The 3 Things To Avoid When Emailing Your List
When you decide to have an opt-in list, it is not just a matter of sending your subscribers your promotional newsletters or catalogs.