Fate and destiny are basically synonymous. The only difference is perhaps that "fate" is an unavoidable circumstance brought about regardless of a person's decisions; whereas "destiny" is an unavoidable purpose achieved through a person's decisions. Both words speak to a predetermined future, but fate pertains to the result of outside influences while destiny pertains to the result of internal influences. But, I would say that any conversation outside of a philosophy classroom can use the terms interchangeably.