CPA means that someone pays when a user completes a more involved transaction. So in the case of an airline, the company pays every time a user clicks on its ad and purchases a plane ticket. Only when both deeds are done does the airline pay, but it will most likely be a larger sum than a simple CPC. Also, CPA has the potential to significantly reduce the occurrence of click fraud because the charge on the advertiser is better tied to the advertiser's desired end result. If CPA becomes the dominant advertising model for Google, it will affect the entire click-oriented search marketing industry.