What is the real nature of one of the most mysterious organizations, the history of which goes back into deep middle ages?

There are crimes, corps and fear which is being represented in the form of indisputable respect for sicilian mafia since long. However, there is no much of reliable data in regard to one of the most powerful world structures. Who, when and for what purpose has formed this structure? What stands behind the triumph of this organization over the period of many centuries?