Pay only for ads that get results? What a novel idea! In reality, not a totally new concept, but something that search behemoth Google has finally decided to deliver upon. Google’s new pay-per-action advertising system allows advertisers to pay only for ads that fulfill a predetermined action on the part of the consumer, be it a sale, a lead, subscription to a newsletter, etc. In short, you decide what you want the ad to compel the user to do and only pay for ads that actually do so. The new system went live for testing on March 20th , and is only available to US advertisers for now.
What this means to you—

Reduced risk. You don’t get charged for online ads that don’t produce the results you’ve defined.
Greater control over your budget. Google allows you to set predetermined costs for each action that your customer takes, based on what value that action has to your business
Open to all (or most) advertising “wallets”. There is no minimum spending requirement to take advantage of this system.
Targeted reach. If you are a very localized or area/specific business, Google is able to set your ads to appear only for people searching in your particular area.
Further reading--
More details straight from Google’s AdWord’s Product Manager- Rob Kniaz.
Andy Beal on Google's new program.