Understanding Web Trackers

Web sites record information about user activity using technologies called “trackers.” The illustration shows how data is tracked, stored, and used.

Why Trackers Are Used

Some of the reasons for deploying trackers are benign, such as recording credentials (so that the user doesn’t have to reauthenticate when navigating to another page on the same site) or a user’s preferences on the site to provide more personalized information. Sites often use trackers to record what products or topics a user might be interested in and can use that information to recommend other products or topics. 

More concerning are advertisers who use trackers as part of their ads (called “third-party trackers” because they are not associated with the site) that can correlate user activity across multiple sites—often without the user’s knowledge, leading to potential privacy violations. 

Tracking Technologies

Several technologies for tracking user activities exist. Transparent pixels placed in Web site images can signal a user accessing the page on which the image is found. “Fingerprints,” consisting of hardware and software metadata (for example, IP address, location, screen resolution, and browser type), can be used to uniquely identify an individual.