Activities (KDE Plasma)
Plasma's attempt to go one step beyond virtual desktops: not just "where are the windows" but "what am I doing" — with its own widgets, windows and recent files per task.
A virtual desktop is a surface; an activity in KDE Plasma is a context. "Work", "home" and "holiday planning" can each have their own plasmoids on the desktop, their own wallpaper, favourite applications and list of recently opened documents, and a window can be tied to one or several activities. The idea was there from KDE 4.0 in 2008, when you zoomed out of the desktop to see all activities as tiles — an interface few understood, replaced after a few releases by a sidebar on Meta+Q. Ivan Čukić built the KActivities framework and the daemon that keeps track of which files and programs belong where.
An activity can be stopped and resumed with its programs, and can be marked private so that nothing in it is tracked. Dolphin, Kate and Konsole are activity-aware; most other programs are not. That is the concept's weakness: the difference from virtual desktops has never been obvious to newcomers, and plenty of people have run a single activity for fifteen years without knowing it. The feature is still there in Plasma 6.