Which is correct: "GNOME Classic" or "GNOME Fallback"?

The classic GNOME 2 interface present in Ubuntu up to 11.04 but not in later releases is just GNOME 2 (with the default shell). The name of the session type for this interface was Ubuntu Classic, and the name of the session type for Unity was (and still is) Ubuntu, but that shouldn't be confused with the name of the interface itself--Ubuntu is an operating system, after all, not an interface.

Nonetheless, since any mention of "GNOME shell" evokes the idea of GNOME 3, and since Unity is also a GNOME interface, it is helpful to be able to characterize the classic GNOME 2 interface as separate both from any GNOME 3 interface and from versions of the Unity interface that used GNOME 2. Thus it's become almost common to call it GNOME Classic.

We should avoid calling it that, because that's capitalized and thus a proper name, and yet that is not the official name for it. In my opinion, we should instead say "the classic GNOME 2 interface."

The practical reason why it is bad to call the classic GNOME 2 interface "GNOME Classic" is that GNOME Classic is the name of the session type in Ubuntu 11.10 (and future versions) that provides the GNOME Fallback interface. The GNOME Fallback interface is a GNOME 3 interface that is designed to look and behave a lot (though not entirely) like the classic GNOME 2 interface.

Here's a condensed list of some interfaces and the session types that provide them in Ubuntu 11.04 and Ubuntu 11.10:

• The Unity interface (sometimes colloquially known as Unity 3D or 3D Unity, as it uses 3D graphics acceleration) is provided by the Ubuntu session type. The overarching desktop environment is GNOME 2 in Ubuntu 11.04 and earlier versions, and GNOME 3 starting with Ubuntu 11.10. The window manager has been compiz for a while. Install this with the ubuntu-desktop metapackage.
• The Unity 2D interface is provided by the Ubuntu 2D session type. The overarching desktop environment is GNOME 2 in Ubuntu 11.04 and earlier versions (but unity-2d was only available by PPA back then), and GNOME 3 starting with Ubuntu 11.10. The window manager is metacity. Starting with Ubuntu 11.04, install this with the ubuntu-desktop metapackage.
• The classic GNOME 2 interface was provided by the Ubuntu Classic and Ubuntu Classic (no effects) session types. The overarching desktop environment was GNOME 2. This could be installed by the ubuntu-desktop metapackage.
• The GNOME Fallback interface (which is a GNOME 3 interface that looks like classic GNOME 2) is provided by the GNOME Classic and GNOME Classic (no effects) session types. The overarching desktop environment is GNOME 3. The window manager is metacity. Install this with the gnome-session-fallback package.
• The GNOME 3 (or GNOME 3 Shell or GNOME Shell) interface is provided by the GNOME session type, starting in Ubuntu 11.10. (In significantly older versions of Ubuntu, the "GNOME" session type had once meant what the "Ubuntu Classic" session type means in Ubuntu 11.04.) The overarching desktop environment is GNOME 3. The window manager is mutter. Install this with the gnome-shell package.
• The KDE 4 (or KDE 4 Plasma, or KDE 4 Plasma Workspaces, or just KDE) interface is provided by the Kubuntu session type. (Considerably over versions of Ubuntu had major version 3 of KDE instead.) The overarching desktop environment is KDE 4 (frequently just called KDE these days, as it came out some time ago). The window manager is kwin (available via the traditional kde-window-manager package and, as of Ubuntu 11.10, also available in kde-window-manager-gles). Install this with the kubuntu-desktop metapackage.
• The Xfce 4 (or Xfce) interface is provided by the Xubuntu session type. The overarching desktop environment is Xfce 4 (also Xfce4 or just Xfce). The window manager is xfwm4. Install this with the xubuntu-desktop metapackage.
• The LXDE interface is provided by the Lubuntu session type; it is also provided with a different configuration by the Lubuntu Netbook session type (this can arguably be considered a different interface). The overarching desktop environment is LXDE. The window manager is openbox. Install this with the lubuntu-desktop metapackage.