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.