A Graphical User Interface to a computer is sometimes called a "desktop." Microsoft Windows, for example, has a desktop that includes a Start Menu, a MyComputer icon, and so on. The desktop is not the operating system. It is only a user interface to the operating system. Linux has a choice of many different desktops. Several of them are installed on the computers in the lab. You can choose the desktop that you want to use when you log in.
Gnome is a full-featured desktop for Linux and is installed as the default desktop environment on the computers in the lab. It's layout resembles the Mac OS/X layout with additional features. At the top left is a series of menus containing various applications. These are part of a "panel" where you can drag icons of oft-used applications if you like. To the right on this panel are various indicator icons and menus for things like logging out or restarting the machine. Everything on this panel is "single-click" activation.
At the bottom of the screen is another panel. This panel will show applications that are currently open. To the right on this panel are 4 little windows. These represent "virtual desktops" that you can click on to get a fresh desktop where you can open more applications. You can then switch back and forth between the virtual desktops to see the windows open on each.
In between the two panels is the actual current "desktop". Programs or downloads that end up here are by default in a "double-click" environment. (However, that can be changed by configuration.)
Gnome is a highly configurable desktop environment, and is the default for Ubuntu installations. Spend some time exploring it's capabilities.
Another advanced and full-featured desktop for Linux is called KDE. KDE stands for the "K Desktop Environment". If you are accustomed to a Windows environment, you should find it pretty easy to use. One thing that you might find annoying at first: KDE is a "single-click" environment -- you only click once on an icon to open it. (However, you can configure KDE to use double-click if you want.)
One feature of KDE is the "Panel" bar that appears along the bottom of the screen. At the left end of this bar is the K menu, which contains many programs and commands similar to the "Start" menu in Windows.
The Panel also generally contains icons for other commonly used programs. It contains two little "desktop" buttons marked 1 and 2. The KDE actually provides these desktops to help you organize your open windows, and you can move between them by clicking on these buttons. You can right click this to configure even more of these desktops. On the panel, you'll also see a section with buttons for all open windows -- click on a button to bring that program's window to the front. And on the right there is a clock. Try clicking on it to see a calendar! Or right-click on it to configure it. (In fact try clicking with the left, right, or middle mouse button on anything you see -- including the desktop itself -- to find out what happens.)
Much of your work in the KDE will be done using either the File Manager or a Console window. You can open a file manager to view the contents of your home directory by selecting the folder icon to the right of the K menu on the Panel, and then clicking the folder icon at the top line. When "Open" appears, click that. Or you can use the lower entries (folders) to navigate to a different subdirectory in your home directory.
KDE is quite complicated. You will learn more about it by experimenting, but you could also try reading the documentation. Just click the life-preserver icon "Help" in the K menu.
Other Window ManagersThere are several other window managers available for Linux. Gnome and KDE are just the two most popular ones and the ones installed on the Math/CS machines. If you decide to install Linux on your own machine, you can install many others (Icewm, blackbox, fvwm, xfce, etc) and then pick whichever one you like best as your default.