A webhook is essentially a user-defined HTTP callback — a simple event-notification system. When a certain event occurs, the source site makes an HTTP request to the URI configured for the webhook. Users can configure them to cause events on one site to invoke behavior on another.
Webhooks are typically used for events like pushing code to a repository or a comment being posted. They're a simple way for web applications to communicate with each other automatically.
For example, a webhook might be used in a scenario where a system logs an error and you want to create a new error ticket in response. Once set up, the process is completely automatic and instantaneous.