An icon factory manages a collection of GtkIconSet; a GtkIconSet manages a set of variants of a particular icon (i.e. a GtkIconSet contains variants for different sizes and widget states). Icons in an icon factory are named by a stock ID, which is a simple string identifying the icon. Each style has a list of icon-factory derived from the current theme; those icon factories are consulted first when searching for an icon. If the theme doesn't set a particular icon, GTK+ looks for the icon in a list of default icon factories, maintained by gtk_icon_factory_add_default() and gtk_icon_factory_remove_default(). Applications with icons should add a default icon factory with their icons, which will allow themes to override the icons for the application.
To display an icon, always use gtk_style_lookup_icon_set() on the widget that will display the icon, or the convenience function gtk_widget_render_icon(). These functions take the theme into account when looking up the icon to use for a given stock ID.