E-mail marketing is based on clicks. This may be a term that is foreign to you if you are new to
e-mail marketing but, for seasoned marketers, their whole day is spent analyzing, buying and
selling clicks.
 
What is a click? And how do you measure the performance of clicks?

E-mail marketers send e-mails to their list (or another person’s list); called a send. A certain
percentage of those people open the e-mails; referred to as opens. Some of the people that
opened the e-mail will click on the link in the e-mail; referred to as a click. A portion of the
people that clicked on the link will click through and optin; this is referred to as a click through
rate (CTR)

Say you have 2 e-mail campaigns; both to 1,000 people. You can compare the CTR of two
different products. It is simplified here for demonstration purpose but, if one had a CTR of 4.5%
and the other had a CTR of 3%, I know which one I would promote.
Solo ads are bought and sold; based on the number of clicks. You measure the responsiveness of
your list; based on clicks.

You must be able to measure the number of clicks that you are getting and that is what tracking
software does for you. 

There are many tracking services available and have different price ranges.