They both mean "still" and are often interchangeable, 여전히 stresses the continuing state.of something. 아직도 may be used like 여전히 but it can also have the connotation that something that should have happened is still not happening, with an imperative sense. I think overall people use 아직도 more than 여전히. - 그녀는 여전히/아직도 천진난만한 소녀 같다. - 5시 도착 예정이었던 비행기가 아직도 도착하지 않았다 (여전히 would be strange).