Pay-per-action advertising is a new pricing model that allows you to pay only for completed actions that you define, such as a lead, a sale, or a pageview, after a user has clicked on your ad on a publisher’s site. You’ll define an action, set up conversion tracking, and create ads that publishers in the Google content network can then choose to place in new ad units on their site. Let’s run through the details: Ad placement: Pay-per-action ads are only shown on sites in the Google content network. Publishers choose specific pay-per-action ads that are relevant to their site and can place them in a new ad unit on their page.
Source: Inside AdWords: Pay-per-action beta test