These are general terms in programming. Often can mean different things but generally speaking...
callback is a reference to a function or block of code that is executed by a third party.
trigger is a behavior in response to stimuli, and an event may
trigger the change of state or as the result of that
trigger execute the associated
handler is a reference to an object or class that is associated with a behavior. A
handler is different from a
callback because it's an object that represents a state.
Using jQuery AJAX as an example.
function() is a
- The object passed to
$.ajax(...) is a
- The event
done() is a
handler has an event
done that when
triggered calls the
callback to perform