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.