The classic Linux desktop -- a workspace, a panel, and a menu -- remains the most popular design for a graphical interface. Unfortunately, however, modern computers include too many applications to fit on a single desktop. Many users respond by launching applications from the menu, often drilling down several levels and sometimes even relying on incomplete menus to reduce clutter. However, KDE offers a more elegant solution in Activities, or multiple desktops, each with its own set of icons.
Activities were introduced with the KDE 4 release series. For some reason, they have never caught on, partly because the project has rarely emphasized or explained them, and partly because how they are different from virtual workspaces has never been clear. It doesn't help, either, that from the virtual workspace pager on the panel, you can set virtual workspaces so that each can have its own icons, widgets and settings, just as Activities can.
Usually, however virtual workspaces are sub-divisions of a desktop, extending workspace without the trouble of setting up multiple monitors. Using virtual desktops, for example, you can keep your browser or email reader open full-screen all the time.
By contrast, Activities are usually organized by tasks or project. By setting up different Activities, you can place all the necessary resources for a task or project in the workspace, a single click away. In effect, they make the classic desktop practical again.
For example, in a general Activity for Writing, you might include a text editor, LibreOffice, a launcher to open the folder where you save your work for the month, and an URL to a dictionary and a thesaurus.
Similarly, for a specific project, you could either link to the project files or use a folder view (see below) so that project folder displays on the entire desktop. For instance, when I was writing Designing with LibreOffice, I had folders on an Activity desktop for chapter files, and another for the top directory for graphics. I placed links to other resources, such as LibreOffice and GIMP, beside them.
I have heard of other uses for Activities, such as a storage space for unread URLs, and replacements for virtual workspaces (no doubt to avoid confusion) but tasks and projects are the uses that are most common. Their advantage is that they make placing icons on the desktop practical again. Unless you are displaying a folder, each Activity generally has less than a dozen icons, which makes resources easy to find. Moreover, having resources a click away lets you find and use them with minimal disruption to your thoughts. By contrast, having to dive into the menu can make you lose your line of thought all too easily.
In the KDE 4 series, Activities also have the advantage of allowing different layouts, including the Default, which is a classic desktop, Folder, in which the entire desktop because the equivalent of a file manager view, Newspaper, which arranges desktop widgets in orderly rows, and Search and Launch, which converts the entire desktop to a menu. These layouts can be set by right-clicking on the desktop and selecting from the drop-down list in the Layout field.
Unfortunately, the developers of the KDE 5 release series seem in no hurry to add this functionality. A case can be made that some of the available layouts are redundant, but the reduced versatility does rob Activities of part of their usefulness.
No matter how you use Activities, their setup is the same. Begin by opening the desktop toolkit in the upper right corner and select Activities > Create Activities. If you don't know what layout you want, you can set it later from the desktop. However, to help you differentiate one Activity from another, choose a distinct icon and name for each. The name will appear on the desktop toolkit when an Activity is the current desktop.
Once you start using Activities, you should also decide how to switch between them. You can use keyboard shortcuts in the menu of the desktop toolkit to move back and forth in the list of Activities, or a widget to open the Activity window.
However, by far the most quickest option is the Activity bar, a desktop widget that lists all the Activities. Like any other widget, the Activity bar can be be stretched and rotated to your liking. Rotating it ninety degrees can be particularly useful, since you can place it on one side of the desktop where it occupies minimal space.
The exact content of each Activity might take some time to decide. In my experience, the more you use Activities, the more new ways you find to use them. However, you might give each Activity its own wallpaper to help you navigate, although the current Activity is highlighted on the Activity bar.
Probably, the number of Activities has an upper limit. However, I keep five general purpose Activities, including one main or default one which would be my desktop on any MATE or any other interface. At times, various projects has raised their number as high as eight. I have never had any problem with them, although perhaps I might if I only one gigabyte of RAM.
What I do notice is that they simply my work-flow, and allow me to work with less distraction. Over the last few years, they have become such a major part of how I work that I grow irritable when forced to work without them on another desktop.
To me, Activities are a successful example of what an update to the classic desktop should be. They still allow you to have a single classic desktop, if that is what you really want. However, Activities also extend the concept of the classic desktop without great altering them. This tactics makes them KDE successful in a way that Unity's over- simplification or GNOME's overview are not.
If you haven't tried Activities, start with one additional desktop and add others as necessary. If you are anything like me, you will probably find them a way to increase your efficiency without having to alter your habits to any extent.