GTK and PYGTK difference

many programmers import both gtk and pygtk in this way:

import gtk
import pygtk

I have created a simple program using only gtk and it works:

import gtk

window = gtk.Window()
window.set_size_request(800, 700)
window.set_position(gtk.WIN_POS_CENTER)
window.connect("destroy", gtk.main_quit)

button = gtk.Button("Vai")
button.set_size_request(30, 35)
button.connect("clicked", naviga)
testo = gtk.Entry()

h = gtk.HBox()
h.pack_start(testo)
h.pack_start(button)

window.add(h)
window.show_all()
gtk.main()

So... the question is: what is the difference from GTK and PYGTK ?