A web beacon is any one of a number of techniques used to track who is reading a web page or email, when, and from what computer. They can also be used to see if an email was read or forwarded to someone else, or if a web page was copied to another website. The first web bugs were small images.

Some emails and web pages are not wholly self-contained. They may refer to content on another server, rather than including the content directly. When an email client or web browser prepares such an email or web page for display, it ordinarily sends a request to the server to send the additional content.

These requests typically include the IP address of the requesting computer, the time the content was requested, the type of web browser that made the request, and the existence of cookies previously set by that server. The server can store all of this information, and associate it with a unique tracking token attached to the content request.