Your app can now hear about opens, clicks, and unsubscribes without polling. Subscribe an endpoint to the new events and they arrive signed and retried, like your delivery events.

message.opened and message.clicked mark the first open or click, and clicks include the link.
contact.unsubscribed and contact.resubscribed fire when someone changes an unsubscribe group, from an email link, the preference page, or the API.

Campaign events tell you which campaign they came from. Keep in mind that opens and clicks are directional: mail privacy features and link scanners can trigger them, and these events don't filter those out.

See Webhook events for the payloads.