Just to sum up the info the other ones said, and adjective describes the object "the rabbit is fast" and "faster" is the comparison... to compare 2 objects with the same adjective as the example they gave you. An adverb describes the action. I run fast, my mom cooks quickly... so words ending with "ly" most of the time are adverbs.

That way to thinking doesn't exist in italian. But to express this way to think I am going to tried to explain the meaning for a non native speaker. faster/fastest is use for comparaison and that rule works for all adjective ending by '"er" and "est" Ex: Your car is faster than my car --> comparaison between both cars.

Fast and faster can be either adjectives (describing things) or adverbs (describing an action or activity) but quickly is only an adverb Fast as an adjective: "That car is fast" Fast as an adverb: "That car is moving fast" Faster as an adjective: "A car is faster than a bicycle" Faster as an adverb: "A car moves faster than a bicycle" *Note that faster is used to compare two or more things Examples of use of quickly: "The car moved quickly" "I ran away quickly"

"Fast" can be an adjective or an adverb (ex. I am fast. I run fast.) "Faster" is the comparative form of fast (ex. Usain Bolt is faster than I am). "Quickly" means the same thing as "fast" but can only be used as an adverb (I run quickly). As an adjective, you would say "quick," however, it carries more a connotation of agility than unidirectional speed.