http://www.gtk.org/tutorial/sec-anupgradedhelloworld.html

[Gert Cuykens <gert cuykens gmail com>]

static void callback( GtkWidget *widget, gpointer   data ){
    g_print ("Hello again - %s was pressed\n", (gchar *) data);
}

why do they put () around gchar ?
why can it not be gchar *data ?

What does %s do ?
Are gpointer and gchar global variables of gtk ?
Are gpointer and gchar data types or objects ?
What does gpointer and gchar represent ?