Web beacons (also known as web bugs) are invisible elements (often transparent 1x1 pixel GIF graphics) on web pages or HTML emails that are designed to inform the originator that an user has viewed the page or email. They are often employed by advertisements publishers to determine the amount of their ad views, or by website to determine their page views.
When used in spam email, web beacons informs the sender that the email has reached the receiver and the receiver has viewed the email. Hence spammers has gotten a confirmation that they have an valid and used email address, which they can then potentially sell to other spammers. Web beacons on email can be prevented by setting your email client or web mail to not display images by default. Then if you have determined that the email is legitimate, you can tell email client to display images for that particular email.
Web beacons work by having an image on the page or email. When the browser or email client renders the page or email, it needs to make a server call to retrieve this image. This request is contained in the web server logs and hence the web server will have information as to when and what IP address has made the request.
