''Quickly'' is an adverb and ''fast'' can be an adverb, an adjective , a noun and a verb. I run quickly. - an adverb I run fast. - an adverb I am a fast runner. - an adjective ''fast'' in a sense of not eating can be both a noun and a verb. He will begin a fast next Friday. He fasts for health reasons once a week.

They are very similar. For example: I ran fast and won the race. I ran quickly and won the race. both mean the same thing. "fast" can also mean not eating. For example: I will fast on Tuesdays and Thursdays to help me lose weight.

Fast is an adjective and quickly is an adverb. Exemple: That car is fast I got here as quickly as possible There is no big difference in the meaning