Web Bug

A web bug (the same term as web beacons, tracking bugs, trackers, tracking pixels, pixel tags, clear GIFs, and 1×1 GIFs) is an object embedded in a web page (usually to track the page's popularity) or e-mail (often to track when an email is read/opened) and is normally invisible to the user but allows checking that a user has viewed the page or e-mail. Web bugs are commonly categorized as Spyware.

Detecting Web Bugs
Without help, web bugs are very difficult to detect and block. The reason is that they are very small (usually 1x1 pixel) and that they are invisible when viewing the web page.

How to Detect Web Bugs
One way to detect a web bug is to look at a page's source code: if you see an IMG tag loaded from a different server than the other ones, then you will know that a web bug is on the page you are on. Web bugs are loaded from a different server so that they are easily distinguished from regular GIFs. You can also use the Ghostery addon for Mozilla Firefox, Internet Explorer, Google Chrome and Opera.

Necessity of a Privacy Policy
If your website uses web bugs, for whatever reason, this needs to mentioned in your site's Privacy Policy.