The Persistent Shadow
Cross-site tracking is the practice of tracking a user's activity across multiple independent domains to build a comprehensive behavioral profile.
The Mechanism: Third-Party Cookies
If you visit news-site.com, and it embeds a Facebook Like button, Facebook (a third party) can set a cookie on your browser. If you then visit shopping-site.com, which also has a Facebook pixel, your browser sends that same cookie back to Facebook.
Facebook now knows you read the news article *and* shopped for specific items, linking the activity.
Cookie Syncing
What if Tracker A and Tracker B want to share data, but they use different cookie IDs for you? They use Cookie Syncing. They redirect your browser between their domains, passing IDs in the URL. Tracker A tells Tracker B: "My user 123 is your user ABC." Now their databases are linked.
State of Defense
Safari (via Intelligent Tracking Prevention) and Firefox (via Enhanced Tracking Protection) block third-party cookies by default, severely crippling this method. Chrome is planning to deprecate them (though timelines frequently shift), pushing the industry toward fingerprinting and first-party data aggregation (like Google's Privacy Sandbox).