When you use CPC ads you only pay when someone clicks on your ad. However, just because someone clicks on ads doesn't mean they will also buy it buy. Therefore, clicks on your ads might not translate to sales. With CPA, people have to take action for you to make sales. In other words, you pay only when someone buys the product you have advertises. Not everyone will do that so conversion is very low. CPM can be good if you can show your ads to people who are really interested in your stuff. It is also comparatively low cost advertising.
