CPM, CPC, CPL, CPA, CPS and CPI are the six most common digital media buying models. CPM refers to cost per thousand impressions, CPC is cost per click, CPL is cost per lead, CPA is cost per acquisition, CPS is cost per sale, and CPI in mobile refers to cost per install. Each model has advantages and disadvantages depending on objectives and what media partners allow. It is important to understand results and quality regardless of buying model to ultimately drive revenue.