At its most basic it is an app launcher. Instead of launching lets say iTunes by clicking on the doc or opening your apps folder you would simply type in a short cut such as 'it'. For those that are more keyboard centric the mouse centric this can be quite a time saver. I began by saying "at its most basic", and the true power of QuickSilver or Launchbar (which I prefer) is that you can extend this power of simply launching an app. For instance you can select some text and in a very few keystrokes open Mail and send that text to someone in your address book. This only scratches the surface of what QS can do, and I'll leave an explanation of "all that it can do" to some one who has truly mastered it.