What is the difference between an event handler and a callback function?
Generally speaking, a 'callback' is under the control of the detecting process.
So you tell a GUI manager "call myaction when this button is pressed" and the GUI manager calls the action when the button is pressed.
Event Handlers on the other hand operate at one step removed. The GUI manager is configured to send messages to an event handler. You tell an event manager that button pushes are handled by the myaction program. When the button is pushed the GUI manager puts a message on the event handler's queue and gets on with GUI managing. The event handler picks up the message from the queue, sees it's a button push, fires up the myaction program, and moves on to handling the next event. Usually the myaction program will run as an independent thread or even a separate process.
While the "event handler" pattern is more complex it is much more robust and less likely to hang when an action fails. It also makes for a more responsive GUI.