The new cost-per-action, or CPA, network will pay only if net users perform a specific action such as making a purchase or generating a sales lead; in contrast, the AdSense model pays based on the amount of clicks a page receives and is particularly susceptible to “click fraud,” the act of clicking on an ad multiple times to generate revenue.
Read this article from WIRED
Plus CPA – The Holy Grail of Online Advertising? from the Read/Write Web (added 9.10.06)
What is “cost-per-action” and “cost-per-click” ? Learn more or add what you know about it to the knowledgebase at the Whats New Media Wiki.