http://www.nytimes.com/2009/03/11/technology/internet/11google.htm
Google will use a cookie, a small piece of text that resides inside a Web browser, to track users as they visit one of the hundreds of thousands of sites that show ads through its AdSense program. Google will assign those users to categories based on the content of the pages they visit. For example, a user may be pegged as a potential car buyer, sports enthusiast or expectant mother.