A small note attached to a web request
The web’s basic request-and-response system has no built-in memory of a person’s previous click. A browser asks a server for a page, receives it, and then makes a separate request for the next page. Without an added identifier, the server may have no easy way to tell that the two requests came from the same browser.
A cookie supplies that missing link. A website can ask the browser to save a small piece of text, often an identifier. When the browser later requests another page from that site, it can send the saved cookie along. The server can then connect the new request with an earlier one—for example, to keep a visitor signed in or remember items in a shopping cart.
How a convenience became a tracking tool
The same mechanism can work beyond the site a person is actively viewing. A page may load an image, advertisement, or other content from a separate company. If the browser has a cookie for that company, it may send it with the request for that content. When the company’s material appears on multiple sites, the identifier can help link visits across them.
That does not mean a cookie automatically reveals a person’s real name or records every action online. It can, however, let a company recognize a browser and build a history of its activity. Advertisers can use such histories to infer interests and choose which ads to show.
The shift came from reusing a practical memory aid in a wider web ecosystem: a note intended to help one site manage a visit could also help another service recognize the same browser in many places. Browsers now limit some forms of cross-site cookie use, but the underlying design lesson remains: a tiny identifier can make separate requests—and separate visits—look like one continuing story.
