The concept of activities is a new feature introduced with KDE 4. In the old desktop model of KDE 3, the desktop was a program called “kdesktop”, which gave users the ability to have a number of virtual desktops. Although other tools like Superkaramba could be used to add more features, the essential KDE desktop ended there.
When activities were introduced into KDE 4, they did not make much sense in isolation. In addition to having virtual desktops, there were activities, which the user could create and configure to have different wallpapers and different widgets. Much of the virtual desktop functionality of KDE 3 was absent and not directly connected to Plasma activities.
With the release of KDE 4.5, Plasma has reached a much higher level of maturity, and activities can now be integrated with virtual desktops, dual monitor screens, and with the Dashboard feature.