@tananaBrian: Hello and welcome to the Gtk+ forums.
GtkTooltip documentation is quite informative about the different ways of playing with tooltips. You can read it here: http://library.gnome.org/devel/gtk/stable/GtkTooltip.html#GtkTooltip.description
GtkTreeView offers some convenience function to work with tooltips. They ar all described in GtkTreeView's API documentation at http://library.gnome.org/devel/gtk/stable/GtkTreeView.html
(the last 5 functions in synopsis part are related to tooltips).
A sample of usage can be seen in one of the evolutions file: http://www.google.com/codesearch/p?hl=en#pzH_xHoYSPE/mail/mail-component.c&q=query_tooltip_cb%20lang:c
Hope you'll find some useful pointers in upper material.