In Adsense, a click is typically counted each time an ad unit is clicked. Since most of the ads in the Adsense network pay per click, the number of clicks will greatly affect how much money a site is making.
The Click-through rate (CTR) represents the number of impressions that has led to a click. It is calculated by:
CTR = Clicks / Impressions
Revenue per 1000 impressions (RPM) represents the estimated amount made for every 1000 impressions. This is calculated by:
RPM = (Estimated earnings / Number of page views) * 1000