Fremantle (Maemo 5) is built on later versions of the same libraries which have been used in Maemo since the 770 was released in November 2005. This includes gstreamer, DBUS, gconf and the Gtk+ widget toolkit. Several UI features in Fremantle will use Clutter (for the first time) to give whizzy dynamic effects. In addition, we'll start seeing more third party apps written using a different GUI toolkit: Qt. They'll still be running on the same OS, though.

Harmattan (presumably Maemo 6) is built on many of the same libraries as Fremantle. However, the roles of Gtk+ and Qt are reversed: the core of the user interface will be written using Qt; though third party apps will still be able to use Gtk+. Similarly, Qt has a technology similar to Clutter for doing fancy transitions - necessary to have a compelling user experience in this day and age.

More information, including links back into the appropriate thread here can be found in this LWN feature article on the subject:

http://lwn.net/Articles/341391/