Some of the most common ads you can use on your sites are CPM (cost per 1000 impressions), CPC (cost per click) and CPA (cost per action). With CPM, you earn a certain amount per 1000 ad impressions, with CPC, you earn from every legitimate click, and with CPA, you earn from every action on your ads (product purchase, for example). Do you use all of these ads on your site? Do you value some ads more than others?