Activities are both KDE's most talked-about and least understood features. Whenever I enthuse over them, I am invariably greeted with so much bafflement that I suspect that they are also KDE's least used features. So, for those who keep asking, "What's the point?" I thought I'd give a detailed description of how I use them.
The typical desktop environment is built around applications, and designed for general purposes. By contrast, KDE Activities are task-oriented, and each one is customized for its specific task, and can have its own layout, widgets, icons, and startup applications. The result is an extension of the concept of virtual desktops (although, somewhat confusingly, each Activity has its own virtual desktops, and you can set up virtual desktops on a single Activity to act like Activities within Activities).
Rest here