Expect to come across this popular question in Google ads interview questions.  CPC (cost-per-click): The charge that Google applies to each click on your ad. Because each click is the outcome of a fresh auction, the cost of each click will be somewhat different. As a result, the phrase "average CPC" is frequently used.  CTR (Click-through rate): The click-to-impressions ratio (CTR = clicks/impressions). For example, if your advertisement was shown 100 times and clicked on 5 times, the click-through rate would be 5%. The CTR is a crucial part of the AdWords system.  CPM (Cost-per-mille): The price for displaying an advertisement 1,000 times (mille is a Latin word that means 1,000). With a cost-per-impression pricing approach, this is more usual. The number of times an advertisement clicks is irrelevant in a CPM system. For advertisements on the Display Network, advertisers have the option of using the CPM pricing model.  CPA (Cost-per-action): A visitor's price to complete a desired action on your website. Advertisers are only compensated when a user converts when using a CPA model. A conversion might be a sale, a lead, a newsletter sign-up, or anything else. This strategy directly links return on investment (ROI) to advertising costs.