A Webhook is a function that occurs when a particular event occurs. It is a method for two systems to communicate using data both systems understand. Unlike an API, a Webhook doesn’t need to be told when to run, it waits for a trigger and then carries out the function with information received from the trigger. 

For example;
A webhook might be connected to a calendar event with the function to send an email to members attending the event. If the owner of that event changes the details of the event the Webhook would trigger when the owner saves those new details.The Webhook would then capture the event’s new details and send the updated information to each attendee.