The Cairo Graphics library is the high performance drawing suite used to do custom drawing in GTK.
ATK is the toolkit that GNOME uses to enable accessibility for users needing extra support to make the most of their computers.
GDK is the low level drawing and events library used by GTK to wrap the underlying diplsay hardware and to react to actions by the user.
GLib is the general-purpose base library in GNOME; most of the its elements are just implementation infrastructure.
GTK is the widget toolkit which is used by applications to create rich user interfaces on the screen.
Pango is the text layout and rending engine used to power font rendering and internationalization in GNOME.