Webhook.

At its core a webhook is a simple HTTP request, usually a POST with a JSON body, that a system sends to an address registered in advance. That address belongs to the receiving system and is often called an endpoint. When an event triggers the webhook, such as 'payment succeeded', 'form submitted' or 'appointment cancelled', the message contains the key data: what happened, when, and which records are affected.

The difference from a classic API lies in the direction. With an API, your own system actively asks: 'Are there new orders?' Doing that every few minutes is called polling; it produces many empty requests, and news arrives late. A webhook reverses the principle: the other system gets in touch as soon as it has something to report. Webhooks and APIs therefore often complement each other: the webhook reports that something changed, and the receiving system fetches the full data via the API when needed.

Typical uses are payment providers informing a shop about successful or failed payments, shop systems such as Shopify reporting new orders to an inventory system, form services, calendars, email delivery services with delivery and bounce notices, and version control systems that trigger a deployment after every push. In automation, webhooks are often the trigger that starts a workflow in the first place.

Because a webhook endpoint is a publicly reachable address, it must be secured. Reputable providers sign every message with a secret key; the receiving system checks that signature and discards anything that does not match. It also holds that webhooks can arrive twice or out of order, and they can fail if the receiver is unavailable at that moment. A robust receiver therefore processes every message so that a repeat does no harm, and additionally reconciles important data regularly via the API.