Webhooks let SaaS B notify SaaS A when something happens in B — without A polling. Examples: 'order placed', 'payment captured', 'WhatsApp message received'. SaaS A registers a URL with B; B POSTs JSON when events occur.
Webhooks are essential for real-time integrations. Without them, you'd poll endpoints every few seconds/minutes — slow + expensive. Webhooks deliver in seconds.
Reliability is the main challenge: networks fail. Good webhooks: signed (HMAC), retried (3-5 times with backoff), idempotent (handle duplicate deliveries), and have an audit log.