빨리 and 빠르게 both mean "quickly, fast" The difference in nuance is in the action taken. 빨리 is a conscious action that is controllable while 빠르게 is out of your control. Eat faster, won't you? 밥 좀 빨리 먹어.(O) 밥 좀 빠르게 먹어.(X) saying 밥 좀 빠르게 먹어 just sounds unnatural because it's a conscious effort to eat quickly. However, making an observation on someone else's action is a whole different story. For example: 너 밥 진짜 빨리 먹는 구나 너 밥 진짜 빠르게 먹는 구나. Both could be used to express "Wow, you really eat quickly". 빨리 <=> 천천히 빠르게 <=> 느리게 Also refer to http://koreanselfstudyisntlame.blogspot.kr/2010/10/vs.html

as far as I understand it 빨리 is more of an adjective, and 빠르게 is an adverb. In writing or on the news, in formal situations, they probably wouldn't use one of them, but I'm not sure which. As far as use and being understood, I think they are pretty much interchangeable. To me it is similar to people talking like this in English: "How are you doing?" "I'm doing good" The grammatically correct response would be "I'm doing well," but 'good' can be perfectly understood as an answer. It may not be correct, but it works

The word 빨리 is an adverb, meaning 'quickly' or 'fast'. Whereas, the word 빠르-게 is an adverbial, derived fron the adjective 빠르다, meaning 'speedily' or 'fast'.