Webhook is HTTP push API, delivers real-time call information to other applications. It is an incredibly useful and a resource-light way to implement event reactions.
Web hooks provide a mechanism where by a server-side application can notify a client-side application when a new event (that the client-side application might be interested in) has occurred on the server.
Webhooks are also sometimes referred to as “Reverse APIs”. In APIs, the client-side application calls (consumes) the server-side application. Whereas, in case of web hooks it is the server-side that calls (consumes) the web hook (the end-point URL provided by the client-side application), i.e. it is the server-side application that calls the client-side application.