> For the complete documentation index, see [llms.txt](https://www.postbackurl.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://www.postbackurl.io/frequently-asked-question.md).

# 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.