I had always heard this word during Yahoo or Google search and noticed the different option provided such as change in the color for selected text etc. I knew how these are implemented using the css and cookies however what I didn't know that this was called as Web beacons or web bugs (generally the latter).
The same concept is also implemented in the spam mails where in user would like to know as who read the mail before deletion. In case if you disable the cookies, the application will term you as a anonymous user.
