Check out http://majorsilence.com/pygtk_book for the most up to date version.
The best way to describe clutter is to quote its home page which states:
Clutter is an open source software library for creating fast, visually rich and animate graphical user interfaces.
Clutter can be integrated with many Linux technologies including GStreamer, Cairo a GTK+. It also is portable and runs on Windows and OSX, which allows for cross platform goodness.