Web beacon

A web beacon is an object embedded in a web page or email, which unobtrusively (usually invisibly) allows checking that a user has accessed the content. Common uses are email tracking and page tagging for web analytics. Alternative names are web bug, tracking bug, tag, or page tag. Common names for web beacons implemented through an embedded image include tracking pixel, pixel tag, 1×1 gif, and clear gif. When implemented using JavaScript, they may be called JavaScript tags. There is a work in progress to standardize an interface that web developers can use to asynchronously transfer small HTTP data from the User Agent to a web server that call it simply beacons (in the context of web development) which can be used to send data to a web server prior to the loading of the document without delaying the load and affecting the perception of page load performance for the next navigation.

A web beacon is any of a number of techniques used to track who is reading a web page or email, when, and from which 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 beacons 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.

Web bugs are typically used by third parties to monitor the activity of customers at a site.

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Wikipedia