What is GTK?
GTK, or the Gimp ToolKit, is the UI toolkit around which Gnome applications are developed.
Gnome is the default UI in Fedore and Ubuntu linux.
What is PyGTK?
PyGTK is a python wrapper for GTK2. Allows for leveraging gtk widgets (such as Webkit) against Python applications. Writing apps to use CouchDB? You can use couch widgets in PyGTK to list the contents of your Couch database. Writing apps that are Dbus aware? You can build guis for them or test them with PyGTK.
PyGTK will take you from building simple scripts to building full-blown applications. PyGTK works with Glade, so you can build your apps in a proper UI designer in lieu of having to scratch everything out in code, saving time and enabling you to GTD (get things done).