What is a webhook?
One service sends an HTTP request to your app the moment something happens.
Say you need to know when a payment goes through. You could ask the payment company every few seconds. Or they could just tell you.
What it is
A webhook is when one service sends an HTTP request to your app the moment something happens. It's an automatic notification between programs.
How it works
You give the service a URL in your app, and choose the events you care about. When a payment succeeds, it posts the details to that URL. Your app checks that the message really came from them, usually with a signature, then reacts: send the receipt, update the order.
Why it matters
The alternative is polling, asking over and over. Webhooks are faster and waste far fewer requests. Stripe, GitHub and Slack all use them.
