Originally they named hurricanes after numbers. For example Hurricane 34567 or Tropical Depression 4569. But then they figured out that not many people, at least in the U.S. pay attention to a series of numbers. They then started to name them after the alphabet, more specifically common names. The names go through a rotation, one set of names will be used one year, then the year after a new set will be used. They can retire the names of hurricanes, Katrina has been retired as a hurricane name.