Together, GNOME 3, KDE, and Unity probably account for at least two-thirds of Linux desktops. However, each of the three offers a desktop experience that differs strongly from the other two, and nowhere is that difference stronger than in the use of virtual desktops. In fact, few other features show so clearly the design philosophies behind the three desktops.
Virtual desktops go by a variety of names. Unity and the GNOME 2 series of releases call them workspaces, while GNOME 3 calls them activities. KDE offers activities, each of which can be divided into separate virtual desktops. However, all the names refer to the same basic concept: additional spaces that you can use to reduce the clutter on your screen and organize your open windows.
New users often overlook virtual desktops, since they are not standard with Windows, but some experts work with them routinely. For example, you might open a virtual terminal in one, an email reader in another, a browser in a third, and the windows for their current project in a fourth. Usually, an applet or taskbar allows you to switch between virtual desktops, as well as keyboard shortcuts.
However, although the basic idea remains constant across different desktops, their implementation is radically different, depending on which of these three desktops you choose.
In the GNOME 2 series, workspaces were available in most default installations, but the decision of whether to use them was up to you. By contrast, in GNOME 3, activities are unavoidable.
Sooner or later, you must click the Activities link on the left side of the panel and open the overview screen, two-thirds of which is devoted to display the activities that are running. Basically, the designers of GNOME 3 have decreed that users will use activities, regardless of whether they want to or not.
Many users might find this decision cavalier. How dare the desktop decide how they should work? But if you can get past this reaction, in many ways, GNOME 3's implementation of activities can improve your work-habits, particularly if you habitually open a dozen or more windows at a time.
Although switching to the overview is a pain in itself, once you are there, you might as well reduce the clutter of windows on your desktop by opening some in a new activity or two. And the overview does make opening a new activity easy -- all you need to do is click on the blank one at the bottom of the display on the right side of the screen.
In addition, the overview displays the open windows on each activity in tidy rows. So long as you have no more than four or five windows open on each activity, this arrangement can help you quickly locate applications.
Lamentably, though, those tidy rows have no relation to the actual state of windows on each activity. For this reason, moving from an activity in the overview to its actual desktop can be disorienting. You can't even change the current window from the overview, which makes the overview less useful than it could be.
For mouse-users, using activities also means that you are switching to the overview more. For fear of repetitive stress injuries from the extra clicking, you will probably want either to learn the keyboard shortcuts for switching between activities and to the overview, or else to define your own.
Like the virtual desktops in earlier editions of KDE and GNOME, activities in the KDE 4 series are available for those who want them, and easy to ignore for the many who see no reason for more than one desktop. But, for those who do want activities, KDE's implementation is far more advanced than the equivalents in GNOME 3 and Unity.
Apparently, the intention in KDE is that users will organize themselves by tasks, with one activity for each task (or, perhaps, on a laptop or netbook, one activity for each location, such as the home or the office).
You can select or manage activities by clicking the desktop toolkit (aka "the cashew") and clicking Activities. In early versions of the KDE 4 series, activities were displayed in an overview like GNOME 3's, which was confusing to many because it was undocumented, and required a lot of zooming in and out. But, mercifully, in recent releases, activities display in a scroll across the bottom of the screen.