I see many of the sites like yahoo mail are using a technology "Web beacon". (You can see this in their privacy/private policy statements). I couldn't get much information about this. All i know is, these Web Beacons are tiny one pixel images used to track the user activity in a page such as how many time he accesses the page.
I would like to know where can i get more information on Web Beacons and can I use it in my programs too.