The other day I did  a search for a birthday present and the next thing I know every page you go to has an AD for the search I just entered.
This is achieved by putting a  cookie  associated with your browser and tracking your digital footprint. In today’s world that footprint is more like a body suit.
Google has an Opt OUT plugin for your browser to remove the cookie



Save your opt-out preference permanently
With this browser plugin you can permanently opt out of the DoubleClick cookie, which is an advertising cookie that Google uses. The plugin lets you keep your opt-out status for this browser even when you clear all cookies.


Download the advertising cookie opt-out plugin




Requires Firefox 1.5+. Also available for Internet Explorer & Google Chrome.
Don’t want to opt-out?
Visit the Ads Preferences Manager to edit the preferences that are associated with this cookie.
Open source
The Google advertising cookie opt-out plugin is available as open source on our project code site.