Webhooks are automated messages sent from apps when something happens. They have a message—or payload—and are sent to a unique URL. They are often referred to as "reverse APIs" because they work in the opposite way: instead of your app requesting data from another app, the other app sends data to your app as it happens. In the context of email marketing, webhooks are used to notify you of events such as email opens, clicks, bounces, and unsubscribes.