From: http://www.eff.org/Privacy/Marketing/web_bug.html
What exactly is a Web Bug?

A Web Bug is a graphics on a Web page or in an Email message that is designed to monitor who is reading the Web page or Email message. Web Bugs are often invisible because they are typically only 1-by-1 pixel in size. They are represented as HTML IMG tags.

What information is sent to a server when a Web Bug is viewed?

* The IP address of the computer that fetched the Web Bug
* The URL of the page that the Web Bug is located on
* The URL of the Web Bug image
* The time the Web Bug was viewed
* The type of browser that fetched the Web Bug image
* A previously set cookie value

What are some of the uses of a Web Bug on a Web page?

Ad networks can use Web Bugs to add information to a personal profile of what sites a person is visiting. The personal profile is identified by the browser cookie of an ad network. At some later time, this personal profile which is stored in a data base server belonging to the ad network, determines what banner ad one is shown.

Another use of Web Bugs is to provide an independent accounting of how many people have visited a particular Web site.

Web Bugs are also used to gather statistics about Web browser usage at different places on the Internet.


If you use Firefox you can actually detect web bugs:

1) Install greasemonkey: http://greasemonkey.mozdev.org/ and restart browser
2) Goto tools -> new user script

then paste this in there (right after // ==/UserScript==), save it (might wanna retitle the line @name bug detector), then open it with firefox and Goto tools -> install user script

------------------- after this line ----------------------
(function()
{
window.addEventListener("load", function(e)
{
var imgList = document.getElementsByTagName("img");
for (i=0; i < imgList.length; i++)
{
if (imgList[i].width == 1 && imgList[i].height == 1 && imgList[i].src != "")
{
imgList[i].width = "101";
imgList[i].height = "101";
imgList[i].alt = "WeB BuG";
imgList[i].border = "7";
imgList[i].style.borderColor = '#ff0000';
imgList[i].style.backgroundColor = '#00ff00';
}
}
return;
}, false);
})();
-----------before this line ----------------
I wrote this myself, with a lil help from a few other sites (Upon loading a site it gathers a list of all objects tagged "img" that are 1 x 1 pixels in size and that have a url associated with them, then it makes them a lot more visible )

This produces a green box 101 x 101 pixels with a 7 pixel RED border that marks the bug ... when you right-click -> Propeties it should say "Alternate text: WeB BuG"
3)Use the Adblock extension to block the image
https://addons.mozilla.org/extensions/morei...ication=firefox
restart browser, right-click the green square and Adblock image ... some of the 1x1 images are spacers for formatting purposes ... you should check the page source to make sure, but blocking the image does not affect site layout, so it is safer to just block them

4)Try it out at http://www.nytimes.com/ a.k.a web bug central
or http://www.cnn.com/

An improved version that allows for image pre-loading ... both work, it just depends on whether you want pre-loading or not (image pre-loading is when a website resizes an image to 1 x 1 pixels in order to supposedly speed up your surfing on their site by having you pre-load the images while viewing the page (like did you ever notice that with some sites your browser does not say that it is 'done' until you have already looked over most of the page ... chances are you were pre-loading an image ... which does speed things up in a way)