PPT Slide
Event - sent to your program when a hardware action such as a keystroke, movement of the mouse, pressing of a mouse button, takes place or when X window changes occur in the display (window appears, window is uncovered, etc)
Signal - Events are translated into signals. They are sent to your program when one or several hardware actions take place or when certain things occur within a widget (scroll bar moves, button is pressed)
Callback function - a function which is called by your program upon receipt of a particular signal.