motorcycle vs dirt bike

motorcycle and dirt bike both are nouns.

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