The CPA ad model is a very commonly used one in affiliate marketing. In this model, advertisers pay publishers only if a user takes a specific action after being referred from their site. This action could be requesting a call back, signing up for a newsletter or making an actual purchase. The price paid for referrals that convert is called the cost per acquisition, and is typically much higher than what advertisers would pay for mere clicks in a PPC ad model.
Google have just announced the launch of their own pay per action advertising model, where advertisers will be pay only for visitors that perform a pre-specified action on the site, such as a sign-up or purchase.
Ads from the new platform will only be shown on the content network, in text, image or text-link ad format. The beta test is only open to current advertisers from the U.S.A. who use conversion tracking.