Web Beacons are usually small images, around 1x1 px in size. They are used to monitor the behavior of users. Some advertising networks may use them in the process of discovering click-fraud. And to display the most relevant ads, based on users past searches, and/or page visits.
How They Work
When the browser request the file, usually an image. It will be possible for the server of which its located, to feed the browser with cookies, as well as log the ip address of the user, along with date and time for the request. It will also be possible to record the Referrer header, which normally is sent by the browser. Which makes Web Beacons an efficient tool for displaying ads.
The Web Beacon might pick up search strings from the major search engines, which are stored in the Referrer. And then use them in attempts to show more relevant ads.