Nothing. They are absolutely the same thing. They are actually called tropical cyclones. When a tropical cyclone is weak, it is called a tropical depression. If it speeds up to 39 miles per hour, it becomes a tropical storm. Should this storm then speed up to 74 miles per hour, it is classified as a either a hurricane, typhoon or tropical cyclone. But they are called by different names to identify what part of the world they occur in. Example, in the north Atlantic, central North Pacific and eastern North pacific, it is called a hurricane. In the north west Pacific, it is called a typhoon. And in the South Pacific and Indian Ocean, the generic term tropical cyclone is used.