Hi! I have just started to learn about IM, but the acronyms make me problems. Ok, lets start... There a two major ways to earn money with ads: CPI and CPA 1. CPI (Cost per Impression) The viewer of the site has nothing to do (no interaction between viewer and ad needed), the owner of the site has become money to display ads for some amount of impressions (see 1.1. etc.). The behavior of the visitor is not important. So, if the CPI is 1 USD, one impression costs 1 USD. 1.1. CPM (cost per mile [= 1000 impressions]), also called CPT (cost per thousand) CPM is nothing else than CPI, but from a relative perspective. So if somebody has a CPI 1 USD, one impression costs 1 USD. But, if somebody has a CPM of 1 USD, one impression costs 0.001 USD. 1.1.1. eCPM (effective cost per mile) The difference between CPM and eCPM is that the whole amount of impressions is considered. Imagine you have 1.000.000 impressions/month. You have become 5000 USD from the advertiser. The CPI would be: 0.005 USD. The CPM would be CPI * 1000, which are 5 USD. For me it seems that CPM and eCPM are in reality the same thing- only in theory different. If I have 1000.000 impressions, I cant take 1000 of them for an adequate analysis (=CPM). I can only consider all impressions. 2. CPA (cost per action), often named PPA (pay per action) and CPC (cost per click) Action matters: The behavior of the visitor is important. He has to CLICK on the ad. It is a strategy which builds on the active interation between ad and vistor. If the visitor of your site clicks on an ad and comes to the site of the advertiser, first you get cash because your visitor has clicked the ad on your site, but its just the beginning: You can earn extra money if your visitor buys something on the site of the advertiser (CPS; Cost per Sale) or if a registration follows after the click (CPL; Cost per Lead). 2.1. eCPA (effective cost per action) I am not sure, because the acronym is not accurate as eCPM, but I think the impressions play again a role, which make CPA relative (as they do with CPI to eCPM). - A CPA is 1 USD: You get for one click 1 USD. - An eCPA is 1 USD: If you have 10000 impressions/month, it would be 0.0001 USD / click. CPI and CPA can also be combinated. Ok, thats my current knowledge about acronyms of IM, which I want to share with newbies like me. I have searched for hours, but what is PTC? Please correct me if something is incorrect.