Google, with its deep reservoir of data about online behavior gathered by tracking hundreds of millions of computers, is for the first time testing ways to use some of that data to aim ads at Web users.Ads that a person sees on one Google search may be influenced by what was searched a few minutes earlier. Searching for “scuba,” then something else, and then “vacations” could pull up ads for diving trips, for example.
http://nytimes.com/2008/06/27/technology/27google.html