Signals are notifications emitted by widgets.

When programming Graphical User Interfaces (GUI), it is often necessary to respond to actions performed by the user or initiated within the program itself. GNOME and GTK+ do this via the use of signals. Signals are used to let the program know that something happened. This might be, for example, a user clicking on a GtkButton, or a change being made to a GtkAdjustment value.

A program may, for instance, cause GtkAdjustment to emit its "value-changed" signal when a widget changes one of its adjustment values. This particular signal is used both internally and externally in GtkAdjustment, so that it is not always necessary to write that signal into your code for, say, a GtkProgressBar to use it. A more obvious situation may occur when a user clicks on an instance of GtkButton, causing the button widget to emit the "clicked" signal. As a programmer, you can set up callbacks to react to any signal emitted by a widget.