Webhooks are custom callbacks (automated messages) sent from a web page or app when they a triggered by an event, such as, receiving a phone call or an incoming SMS, and provide a method for augmenting or altering the behaviour of the web page or app. They contain a message or payload and are sent to a unique URL, essentially the app's phone number or address. Callbacks may be maintained, modified, and managed by third-party users and developers who may not necessarily be affiliated with the originating website or application.
Extensible Markup Language - This is a standard for creating markup languages which describe the structure of data. It is not a fixed set of elements like HTML, but rather, it is like SGML (Standard Generalized Markup Language) in that it is a meta-language, or a language for describing languages. XML enables authors to define their own tags. XML is a formal specification of the World Wide Web Consortium.