A Webhook is like a REST API in reverse. Just like a REST API, there's a specification for a payload: what data will be included in the HTTP request. But instead of us giving you a URL the URL to send it to, you give us the URL where it should be sent. They're a way for us to push data to your web server as needed. Examples include having a call detail record pushed to you when a call ends, or getting notification when an SMS message is delivered to the user.
Webhook subscriptions can be created and managed by logging into the Tropo web console or via a REST API.