Here's where the concept of a desktop environment kicks in. The idea is that a desktop environment provides a set of facilities and guidelines aiming to standardizing all the stuff we mentioned so that the problems we mentioned earlier are minimized.
The concept of a desktop environment is something new to people coming for the first time to Linux because it's something that other operating systems (like Windows and the Mac OS) intrinsically have. For example, MacOS, which is one of the earliest graphical user interfaces, provides a very consistent look-and-feel during the entire computing session. For instance, the operating system provides a lot of the niceties we mentioned: it provides a default file manager (the finder), a systemwide control panel, and single toolkit that all applications have to use (so they all look the same). Application windows are managed by the system (strictly speaking there's a window manager working there). Finally, there are a set of guidelines that tell developers how their applications should behave, recommend control looks and placement, and suggest behaviors according to those of other applications on the system. All this is done in the sake of consistency and ease of use.
This begs the question, "why didn't the X developers do things that way in the first place?". It makes sense; after all, it would have avoided all the problems we mentioned earlier. The answer is that in designing X, its creators chose to make it as flexible as possible. Going back to the policy/mechanism paradigm, the MacOS provides mostly policies. Mechanisms are there, but they don't encourage people to play with those. As a result I lose versatility; if I don't like the way MacOS manages my windows, or the toolkit doesn't provide a function I need, I'm pretty much out of luck. This doesn't happen under X, altough as seen before, the price of flexibility is greater complexity.
Under Linux/Unix and X, it all comes down to agreeing on stuff and sticking to it. Let's take KDE for example. KDE includes a single window manager (kwm), which manages and controls the behavior of our windows. It recommends using a certain graphic toolkit (Qt), so that all KDE applications look the same, as far as their on-screen controls go. KDE further extends Qt by providing a set of environment-specific libraries (kdelibs) for performing common tasks like creating menus, "about" boxes, program toolbars, communicating between programs, printing, selecting files, and other things. These make the programmer's work easier and standardize the way these special features behave. KDE also provides a set of design and behavior guidelines to programmers, with the idea that, if everybody follows them, programs running under KDE will both look and behave very similarly. Finally, KDE provides, as part of the environment, a launcher panel (kpanel), a standard file manager (which is, at the time being, Konqueror), and a configuration utility (control panel) from which we can control many aspects of our computing environment, from settings like the desktop's background and the windows' titlebar color to hardware configurations.
The KDE panel is an equivalent to the MS Windows taskbar. It provides a central point from which to launch applications, and it also provides for small applications, called "applets", to be displayed within it. This gives functionality like the small, live clock most users can't live without.
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