what are the differences between Gnome 2 and Gnome 3? And what is the Gnome Shell exactly, and is it possible to use Gnome 3 without the Gnome Shell (or are they the same thing)? And if so, would it be possible to maybe develop another shell similar to Gnome Panel?
GNOME3 is the next major version of GNOME with lots of changes. It features a new desktop shell, libraries with new API, deprecation of a few libraries, rewritten applications like power-manager, introduction of gnome-control-center etc.
GNOME 3 was like - "GNOME2 was released years and years back, let's start working on the next generation GNOME desktop and hence you find GNOME3"
Gnome shell is a desktop shell for GNOME. It is the UI part of the desktop. Like in Windows you have a taskbar below, with a start button on the bottom left side and icons on the desktop. The thing which you encounter after you login.
The shell decides how your notifications should show up, how your running applications should be shown and how new applications can be launched.
Unity present in Oneiric will be going to be ported to Gtk3 and GNOME3 technologies. I am running Oneiric development version and get many of the GNOME3 goodies like newer applications for power management, mouse & keyboard settings.
So basically Oneiric can be thought of as GNOME3 - GnomeShell + Unity (some more changes under the hood which is out of scope of this explanation)
Technically speaking, GNOME3 is a suite of technologies, applications, libraries, desktop-shell. Gnome-shell is one part of the whole GNOME3 stack
Actually you can still have GNOME Panel for GNOME3. Actually it is GNOME Shell which looks and behaves like GNOME Panel. Vincent Untz blogged about it earlier
It would also be worth noting that desktop shell and window managers are separate things. Desktop shells are written over Window manager. KDE's Window Manager(WM) is KWin, Unity's WM is Compiz 0.9 (not Compiz 0.8) and GNOME Shell WM is Mutter (Metacity+Clutter). Gnome panel's WM is Metacity
In fact you can use GTK3 and use GNOME3 technologies write your own shell with the WM of your choice and it would still run. The whole Linux stack is pretty much flexible and many components are be added and removed at your will.
You can replace Mutter and Gnome Shell with about any other window manager and panel or dock or whatever you want.
GNOME3 comes with a fallback mode that is based on Metacity and Gnome Panel, quite similar to GNOME2.
In Ubuntu 11.10 Unity will be based on Gnome3, so together with Compiz (the window manager for which Unity is a plugin) it will replace Mutter and Gnome Shell in the Ubuntu standard installation.