AdWords ads appear on Google.com as well as on Google partner sites like Ask.com. AdWords ads may also appear on third-party Web sites which use AdSense, the other side of the Google advertising model.
Google charges its advertising customers when someone "clicks" on one of their ads. During the sign-up process, users tell Google the maximum that they are willing to pay per "click."
During this process, users encounter two adjacent boxes. Into the first, customers enter the amount they wish to pay per click of an ad displayed on Google.com. The second box is marked 'optional.' Into this box, a user can enter the amount they would be willing to pay per click of an ad appearing on a third party web page. But leaving the box blank does not prevent ads from appearing on third-party sites.
Google places the ads on third-party sites anyway. And users are automatically charged per click based on the amount they entered into the first box. There isn’t a clear explanation, so make sure you enter the amounts you want to pay in both boxes.