Once upon a time there was a difference, but that difference has been lost. A skillet has either long or short legs so it can stand among the coals of a hearth. Another name for this item is spider, though I can't recall ever actually hearing that name used.
Frying pan, as Tdol mentioned, is the correct term for the utensil we use today for frying, though many will use the word skillet to mean the same thing- especially one made of iron.