A webhook is an HTTP callback: when an event happens in System A, System A makes an HTTP POST to a URL the customer configured. Webhooks are how SaaS applications notify other systems of events in real time, without those other systems having to poll.
Production-grade webhooks include: HMAC-SHA256 signatures so receivers can verify the payload originated from the sender; exponential-backoff retries when the receiver returns 5xx; a replay endpoint for re-delivering missed events; and an idempotency mechanism (event IDs) so receivers can safely handle duplicates.
For document workflows, common webhook events include: link visited, envelope signed, proposal accepted, contact updated, room invitation accepted.