Good question! A tracking pixel is actually a script that captures data about a web site visitor and finally outputs a 1×1 pixel transparent GIF image.

Any web page can host the pixel since it is called by a simple HTML image tag.

The name of the pixel is not “pixel.gif” but rather, something like “pixel.php”. The pixel is transparent so it doesn’t affect the appearance of the web page.

The name could be “pixel.gif” though. In this case, the website serving up the image would use a method known as URL rewriting to run the script called “pixel.php” rather than looking for “pixel.gif”.

To pass data to the script, a query string is appended to the url e.g. site.com/pixel.php?ref=yahoo&kw=searchterm

The URL of the image is often constructed client-side (using the browser) using Javascript. This will typically add the query string of the page and the referring page data and a tag to identify the page.

So, if you want to track visits to web pages you would typically have a snippet of code to cut and paste to the HTML of that page. Then when visitors land on that page, the browser requests the pixel from the tracking site passing the data to it via the URL query string.

Other ways to pass data to external sites as a page loads involve embedding server-side code into the web page which may not be supported by your hosting, so this is why the Tracking Pixel is preferred since it should work on any web page unless the user has software on their computer to block the pixel.