With pay-per click (PPC) advertising you pay the publisher when someone clicks on one of your ads. Pay-per-impression (PPM) on the other hand costs you money every time someone sees one of your ads. Think of it as paying for engagement vs paying for eyeballs on your ads. Which one you use depends on your circumstances. For example, if you have a clear CTA, PPC is better. But PPM is better if your goal is to increase brand awareness.