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.
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.
A tracking server is used by digital marketers to track, report and attribute conversions.
A webhook is a URL that allows application servers to receive data through query string payload or JSON payload.