Ad Clicks

Ad clicks are often used as a marketing goal; instead of paying for ad impressions, you only pay for clicks — often referred to as cost-per-click (CPC) or pay-per-click (PPC).

This metric is often more expensive than ad impressions because the users are one step closer to downloading your app. Also, in this case, ad network transfers the risk from the developer to itself: the developer only pays for the completed action, while the ad network has to pay for impressions to their suppliers and rely on its own algorithms to make sure that these impressions in fact bring the revenue via clicks.