if rápido is in itself an adverb...
what is rápidamente? Is there a difference?
2 Answers
Yes. Rápido can also be an adjective - quick. Rápidamente is always an adverb - quickly.
As adverbs they're often used interchangeably, but not always.
¡Vamos! ¡Rápido! - Let's go! Quickly!
Caminaron rapidamente por la calle - They walked quickly down the street.
Caminaron rápido por la calle sounds funny to me.
KevinB is right: words ending in -mente are adverbs, and rápido, which was originally an adjective, is also used frequently as an adverb (meaning rápidamente.)