Cost per Thousand Impressions (CPM) is one of the most common pricing structures. The advertiser pays for each time an ad is viewed. CPM models have the benefit of transparency because the advertiser pays the publisher the true price of the inventory.
Cost per Click (CPC) also called pay-per-click (PPC) is a pricing structure in which the advertiser pays a publisher every time an ad is clicked on.
Cost per Acquisition (CPA) is a pricing structure in which the advertiser pays every time a transaction, for example, a purchase or a download, is completed. CPA models can be tricky as, often, it is very complicated to truly track where a purchase was derived. Many CPA based models will take credit for the completion of a transaction that was not truly derived from them.