A web beacon is a small image file — usually a transparent 1x1 pixel image — used for tracking purposes. It may be placed in a webpage or HTML email to record when the content was loaded.
How do web beacons work?
Each image referenced within a webpage is retrieved from a server. When the image is loaded, the server records a "hit," which is typically saved in the server log. A log file on the server can be used to determine when the specific image was accessed and also the IP address that accessed the file.
While any image can be used as a web beacon, small transparent GIFs or PNGs are common since they can be placed unobtrusively on a page. They may also be used by third-party tracking tools that are not accessed from the main web server. Examples include analytics code like Google Analytics and affiliate links provided by other companies. An affiliate link, for example, may include a web beacon before or after the link. The beacon allows the publisher to record the number of impressions (or number times the link is displayed), which is not possible with a plain text link.
Web beacons in email marketing
Web beacons are commonly used in email marketing to track the number of users who open and view emails. For instance, when you view a promotional email, a web beacon hidden in the email may record that you have opened it. This helps email marketers know which campaigns are effective. Unfortunately, email beacons may also be used for spam purposes, as they can record valid email addresses. For this reason, many email clients and webmail interfaces do not automatically load images in emails that are likely to be spam.