Gnome Flashback (previously called GNOME fallback mode) is a session for Gnome 3 which was initially called "Gnome Fallback", and available as a stand-alone session in Debian and Ubuntu. It provides a similar user experience to the Gnome 2.x series sessions. It consists of the following components: GnomePanel, Metacity, GnomeApplets, and etc. It doesn't use 3D acceleration at all, so it's generally faster and less CPU intensive than GNOME Shell/other desktops with llvmpipe.
The GNOME Session Manager is in charge of starting the core components of the GNOME desktop, and applications that should be launched at login time. It also features a way to save and restore currently running applications. This is a transitional package to ease upgrades to gnome-session-flashback. It can be safely removed.
