motorcycle vs dirt bike

motorcycle and dirt bike both are nouns.

Word	Noun	Adjective	Verb	Adverb
motorcycle	Yes	No	Yes	No
dirt bike	Yes	No	No	No

As nouns, dirt bike is a hyponym of motorcycle; that is, dirt bike is a word with a more specific, narrower meaning than motorcycle:

motorcycle: a motor vehicle with two wheels and a strong frame
dirt bike: a lightweight motorcycle equipped with rugged tires and suspension; an off-road motorcycle designed for riding cross country or over unpaved ground

Other hyponyms of motorcycle include minibike, motorbike, scrambler, trail bike.

motorcycle (noun)	dirt bike (noun)
a motor vehicle with two wheels and a strong frame	a lightweight motorcycle equipped with rugged tires and suspension; an off-road motorcycle designed for riding cross country or over unpaved ground

motorcycle (verb)	dirt bike (verb)
ride a motorcycle