Gnome - the 250-pound ape
Gnome is a desktop environment that feels similar to KDE, but is much lighter and friskier. Gnome is second in popularity only to KDE, and is the chosen desktop of Ubuntu, which is currently the most popular desktop-oriented distribution. Gnome has a chummy, comfortable feel to it. If KDE is user-friendly, Gnome is aggressively so! I never saw the Gnome program or feature that wasn't well-documented. For this reason, it is usually the first desktop recommended to newbies, as it is both a very complete introduction to all that FOSS has to offer and very discoverable.
It has everything that you'd expect from a desktop environment, including it's own full suite of Gnome-integrated applications which are identifiable by the letter "g" at the beginning of the program's name. Gnome is the desktop of choice for many, providing a good balance of features and speed. It sticks closest of all to the Unix philosophy: Gnome programs are smaller and lighter, but efficiently well-suited to the task.
Every Gnome program will run happily in any old desktop at all, provided the GTK support library is already installed. This is almost never an issue, as the GTK library is a highly common part of many FOSS programs including staple applications like Gimp. Thus, while I may not use Gnome itself, I have chosen gnome-terminal for a console, prefer gedit for a GUI text editor, and use Gnome applications here and there. It's very rare to see any Gnome program crash - it's rock-solid stable! But on the other hand, it isn't very heavy on features. Even at that, it is the second-slowest FOSS desktop right after KDE, most noticeable on start-up.
Gnome is the desktop environment which is not stand-alone: it needs to host a window manager. In the old days, this was Enlightenment, and Gnome looked fantastic. Today, it's window manager is Metacity, whose own documentation boasts that it's the "boring window manager for the adult in you"; so it's traded eye-candy for speed.
Gnome is recommended for the slightly more discriminating user who is still pretty mainstream in their choices.
For developers, Gnome is even more popular, because it uses the GTK+ library, which itself is the native library of Gimp. While GTK is still a bit of a hassle for making a GUI program in, Gnome's own Glade has got to be the easiest-to-learn IDE for a GUI library ever, so GTK development is mostly painless.
