Callback plugins basically control most of the output we see while running cmd programs. But it can also be used to add additional output. For example log_plays callback is used to record playbook events to a log file, and mail callback is used to send email on playbook failures. We can also add custom callback plugins by dropping them into a callback_plugins directory adjacent to play, inside a role, or by putting it in one of the callback directory sources configured in ansible.cfg.