Look at TCallback - most callbacks are wrapped up with one of them.

Essentially, a static/global function that returns a TInt and takes a TAny*

Well a callback is just a global/static function with a known return type and a known parameter - basically one that can easily be called via a function pointer.  callback isn't a type of function - such as global or member function - its a word used to discrible a function where the caller knows what to pass.

Basically, any global/static function can be a callback - or any memeber function can be with some nasty C++ to call memebers via function pointers which is why members are usually avoided as call backs.  

You pass a pointer to a function to some other function, then later it calls you back - and to do that, it must know what parameters you are expecting - which is what the word callback is used to describe.

I haven't explained that too well, but I hope you get me.