Hi Lasse,
You can take a look at /usr/bin/ansible and /usr/bin/ansible-playbook for examples of how these are used.
Basically some of the ‘single-action’ kind of things are runner callbacks, playbook callbacks are more associated to running the ansible playbook language and higher level things.
You can also see some example callbacks here: https://github.com/ansible/ansible/tree/devel/plugins/callbacks
My favorite is the OS X Say plugin, which makes sound effects as you run your playbooks
I’m curious about what kind of application you are developing!
–Michael