# Frequently Asked Question ## What is a tracking pixel? A tracking pixel is an invisible image pixel that you place on your "thank you" page, and which will inform a tracking server that a visitor loaded the thank you page. The thank you page is usually the page that loads after the visitor completed an action. ## What is a server to server pixel? It's an alias for Postback URL. The name come from the fact that the pixel is called by a server and informs an other server that a conversion has occurred. ## What is a tracking server? A tracking server is used by digital marketers to track, report and attribute conversions. ## What is a webhook? A webhook is a URL that allows application servers to receive data through query string payload or JSON payload. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://www.postbackurl.io/frequently-asked-question.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.