Interest Based Advertising.
Adsense announced today that they will be directing more direct ads to their clients users by getting the niche correctly. Why would you be interested in cars and be getting ads more directed at baby formula.
Marketing companies spend a huge amount of money on these campaigns and for them to go to the wrong audience. They will use a cookie that will save and send what sites you've been looking at a lot.
Inside Adsense quotes:
"To develop interest categories, we'll recognize the types of webpages
users visit across the AdSense network. As an example, if they visit a
number of sports pages, we'll add them to the "sports enthusiast"
interest category. You can visit the Help Center
to learn more about how interest categories will be developed and your
associated account settings. As a result of this launch, your privacy
policy will now need to reflect the use of interest-based advertising.
Please ensure that your site's privacy policies are up-to-date and make any necessary changes by April 8, 2009."
You can read more about how this will affect the nature of your Adsense and more on the official Google Blog.
So please if you have an adsense account make sure the changes are made please.