The desktop toolbox, accessed via the upper right corner has a button for displaying your activities, of which Plasma allows you to have more than one. Basically, that is multiple desktop containments hosting multiple sets of Plasma widgets. Display the “Activities” bar, select one of the predefined activities or choose Create Activity to create a new containment, select your new containment and customize suiting your taste. Plasma’s activities and KWin’s desktop grid are similar in that respect, but there is a fundamental difference. While virtual desktop are used to group and organize windows, Plasma’s activities are used to group and organize Plasma widgets. This way, you can switch between activities and have relevant Plasma widgets supporting the task you are currently trying to accomplish. You can create a “Free time” activity, with comic strips, a puzzle and other Plasma widgets, and a “Work” activity, with relevant RSS feeds, calculator and calendar, for example.
To delete an activity, press the button on “Activities” bar (press Alt+D then Alt+A to open this bar) then click the red 'X' (or press Alt+D then Alt+R) and confirm the deletion.