After testing pay-per-action AdSense ads last year, Google officially launches the program (although it's limited to US advertisers).
"Pay-per-action advertising is a new pricing model that allows advertisers to pay only when specific actions that they define are completed by a user on their site. Rather than paying for clicks or impressions, advertisers can choose to pay when a user makes a purchase, signs up for a newsletter, or completes any other clearly defined action that they choose."
The new pay-per-action ads will have a format similar to the current AdSense referral blocks. "Publishers can choose to place specific pay-per-action ads in new ad units that they create, or allow us to serve the highest converting pay-per-action ads that are related to a topic that they choose."
* formats: text ads, image ads, embeddable links. The text and image referral ads are available in the same sizes as the existing image ad formats.
* ad scope: you can choose a keyword or a list of up to 15 products. Google will rotate the ads and try to display the best performing ones for a site.
The new ads are a departure from AdSense's philosophy of bringing contextual ads, but they give publishers more control over the products that are promoted. The ads can be endorsed, they're more expensive, but require more than a simple click. Visitors should buy a product, fill a form, register to a service, download a program, or perform other actions selected by the advertisers.
To sign up for the AdSense side of the service (you're a publisher), go to AdSense Referrals Beta. To sign up for the AdWords side of the service (you're an US advertiser), go to Google AdWords.