Callbacks and Signal Handlers
Callbacks are functions that are invoked by some other piece of code.
Signal Handlers are callbacks that are executed when a particular signal is
Signal handlers are the places where you do the actual work of acting upon
signals that are received. As a programmer, a signal handler is where you
will place the code that should be executed when a particular signal is
emitted by the widget.
Signal handlers are of two kinds: default handlers and
user-defined handlers. The
default handlers are called automatically on receipt of
their corresponding signals. User-defined handlers on the
other hand have to be explicitly connected to signals
by the programmer (that's you!).