http://adwords.google.com/support/bin/answer.py?answer=61449&topic=11637
Google Pay-Per-Action allows you, the advertiser, to pay only when a customer completes a specific action or goal on your website. The PPA model allows an advertiser to define specific actions or goals and set the rate that they are willing to pay for those actions to occur, rather than paying on a PPC basis. An example of an action could be when user makes a purchase, signs up for a newsletter, or fills out a “contact me” form.
Pay-Per-Action Ads are eligible to appear on the Google Content Network, not the Google search results page. The ads can appear in publisher-defined ad units, in which publishers who participate in the Google Content Network will choose to display the ads they feel will appeal to their sites' users. Ads can also appear in standard AdSense for content ad units, in which the ads will be contextually targeted to a publishers' website and displayed automatically for content ad units. They'll compete in the auction against other ads eligible to appear in these ad units, including Cost-Per-Click and Cost-Per-Thousand Impression Test and Image Ads. This pricing allows advertisers to control their cost per transaction to the fullest extent.
Gazelle Interactive was selected as one of an exclusive set of Google Adwords agencies for Google Pay-Per-Action Ads Beta Program, giving us a heads up on the level technology and services we can provide to you. We facilitate the entire process, from creation to optimization to reporting, just as in all of our other offerings.