KDE is a desktop environment that aims to be the best graphical interface between humans and the information they have stored in computer systems. It aims to not only provide the framework but also the applications. The framework draws on the screen, accepts user input, the applications apply such input to modify the information, and passes the results back to the framework where they are displayed. However, it takes time for a computer to do this work. The time between user input and computer output is called "latency". The longer this time, the less pleasing the experience is for the user.

A second oft-cited concern of users is that buttons with the same functions are located in various locations in the interface. KDE attempts to limit this by providing standards for the makers of the applications so that consistent interfaces are presented to the user.

Consistency is nice, but consistent mysteries don’t help you! KDE tries to be clear in what the results will be of user actions. It does so by providing pictures on buttons (icons) that represent the results of activating such buttons. However, some people don’t quickly understand icons, so straightforward text is sometimes better. KDE allows both and gives users the option of customizing their interface to their information so that they can work as effectively as possible.

And that is the goal of KDE: giving humans an effective interface to work with their information on computers.

Plasma Workspaces

With the development of KDE 4, some major changes were made to the desktop. The first of these is Plasma Workspaces. Unlike KDE 3, the desktop is no longer synonymous with a folder. Instead, it has become an area for displaying small programs called "widgets". (Originally, these widgets were called "plasmoids", but this was abandoned for the more generic term.) Everything on the desktop is a widget — trash can, launchers, task bar, etc. You can easily add pre-installed widgets to the desktop or task bar, or download and install new ones.

You can switch this setting back to the old desktop behavior.

Right-click on the desktop, and select Desktop Settings.
In the View section, under Layout, select Folder View. Now, the desktop will display the contents of the ~/Desktop folder.

To add a widget to the desktop:

Open the Widgets menu by right-clicking on the desktop and choosing Add Widgets. (You can also get to this menu by clicking on the three dots at the edge of the task bar or in the upper left-hand corner of the desktop.)
A searchable menu will appear. To add a widget, click on it and drag it to the desktop or task bar.
To install a new widget, click on the Get new widgets menu at the bottom. Then, choose Download New Plasma Widgets or Install Widget From Local File.

Clicking on Download New Plasma Widgets opens the Plasma Add-On Installer. You can use this utility to install or uninstall widgets. It is a searchable browser of widgets available on store.kde.org. You can sort by Rating, Newest, Most downloads, and Installed. To get more information on a widget, click on its Details button, or click on the blue link to open its KDE Store page in a web browser. When you've decided which widget you want to install, click Install. That widget will now be available to you from the Widgets menu.

Activities

The second big change is Activities. KDE 3, like other desktop environments, allowed you to have multiple desktops. Some users found it convenient to have one set of programs open on one desktop, and a different set open on another desktop. KDE 4's Activities extends upon that compartmentalization even further. Instead of just having separate groups of programs, now you can have entirely different desktops...with different plasmoids, wallpaper, and icons. Additionally, each activity can have multiple desktops, used for grouping together open applications.

To enable different widgets for each activity:

From the K Menu, click on Settings > System Settings.
Under Workspace Appearance and Behavior, click on Workspace Behavior.
Go to the Desktops tab, in the Virtual Desktops section. Under Layout, check the checkbox for Different widgets for each desktop.
Click Apply.

The Activity Manager is a plasmoid that lets you create, start, and stop activities, as well as switch between open activities. You can click on the red X in the upper right to delete an activity, or click on the wrench in the lower right to configure the activity. To change an activity's icon, click on the wrench, and then click on the icon panel.

To switch between open activities using the keyboard, use Super-Tab to go to the next activity, or Super-Shift-Tab to go to the previous activity. The Activity Bar plasmoid is a panel that has a button for each open activity. You can switch to an activity by clicking on its button.