Webhooks provide a functionality where by a provider application can notify a client-side application when a new event has occurred on provider's server.
Webhooks are "user-defined HTTP/S callbacks", that allow two applications to communicate automatically.
A key difference between an API endpoint and a webhook:
In APIs, the client-side application calls the server-side application to retrive some sort of information. Whereas, in the case of webhooks it is the server-side that calls the client-side application, when a particular server-side event occurs.