Etags use persistent identification elements (PIE) that have been tagged to the user’s browser. Although a user may remove HTTP cookies, ETags store the same information along with covert backup to reconstitute the data of deleted cookies.
Most typically, ETags request Web resources on the condition that they have been updated since the user’s last visit to the site. For instance:
A user may visit a site featuring a background that changes every week. On the first visit in a new week, the browser checks the cache and, finding no image or an outdated one, downloads the current background and caches it. If the user had already visited the site that week, the browser would receive the return response that the image had not changed. In that case, the browser would use the local copy in the cache, saving bandwidth and speeding load time.
HTML5 local storage and cache cookies enabled through ETags are also one method used to respawn cookies. Respawning cookies are used by consumer tracking firms to address the issue of user deletion or denial of cookies. The practice can also be used to track users for other purposes, such as keeping tabs on hackers.