Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
No feature defines the KDE 4 release series more than Activities. At the same time, no feature is so little understood -- Fedora even has a package for removing the desktop toolkit, which provides mouse access to Activities.
But, when you take the time to learn about Activities, you'll find them a natural extension of the desktop metaphor that just might help you to work more efficiently.
Activities are a super-set of Virtual Desktops. They don't replace Virtual Desktops -- in fact, each Activity can have its own set of Virtual Desktops if you choose. Instead, Activities are alternative desktops, each of which can have its own wallpaper, icons, and widgets.