Callback Documentation/Tutorial

HI Guys,
Can anyone point me towards a tutorial on callbacks or explain to me how to use them.
What I want to do is if the player collides with an item(not the floor,not a bullet,not a platform) apply an effect to him such as increase his health(undecided as of now).
I've spent the day googling and am quite confused with flags and groups, I dont really understand what is going on
Regards,
Ed

For BloodyMes have a look at this tutorial: BloodyMess Tutorial 4. I explained there how Volumes work. Your problem is quite similar. You also need a callback class with at least one collision handling function (in the tutorial it is onVolumeEvent, yours would be on ContactEvent) and assign it to your Actor.

In Detritus, for some reason it isn't working yet (at least for me).

Thanks for your help.
I have another question for an array of enemies, do I set up a contact pair with the player for each enemy?