Web bugs are methods for spammers to validate email addresses. A small graphic inserted in a spam mail, notifies the spammer when the messages is read or previewed. This is why we recommend that you disable the preview function in your SPAMfolder. This way you do not accidentally open a spam mail and verify your email. This is also known as Bacon URL.
A 1 pixel-by-1 pixel image tag added to an HTMLmessage and used to track open rates by email address. Opening the message, either in the preview pane or by clicking on it, activates the bug and sends a signal to the Web site, where special software tracks and records the signal as an open.
A graphic on a web page or in an email message that monitors who is reading the web page or email message. Web bugs are often invisible because they are typically only 1x1 pixel in size. They are invisible in order to hide the fact that the monitoring is taking place.
Some Web sites employ what are known as "clear GIFs" -- invisible page elements that first came into general use as a means of adjusting the formatting on Web pages. As Richard M. Smith of the Privacy Foundation has shown, these devices can be used as "bugs"; that is, they can be used to track the surfing behavior of anyone who downloads the page they're on.
Web bugs are an acknowledged tool used by an advertising services company that fetches data from multiple Web sites without the users' knowledge and sends the information to its server databases for tracking users browsing habits, analysis, and storage. Web Bugs are in Web pages, email messages, and are of concern regarding security or privacy on the Web. Web bugs are typically invisible to the user as tiny transparent images in a web page.
A Web bug is an object that is embedded in a web page or e-mail and is usually invisible to the user but allows checking that a user has viewed the page or e-mail. One common use is in e-mail tracking. Alternative names are Web beacon, tracking bug, pixel tag, and clear gif.