swimmer vs backstroker

swimmer and backstroker both are nouns.

Word	Noun	Adjective	Verb	Adverb
swimmer	Yes	No	No	No
backstroker	Yes	No	No	No

As nouns, backstroker is a hyponym of swimmer; that is, backstroker is a word with a more specific, narrower meaning than swimmer:

Other hyponyms of swimmer include breaststroker, diver, plunger, skinny-dipper.

swimmer (noun)	backstroker (noun)
a trained athlete who participates in swimming meets	someone who swims the backstroke
a person who travels through the water by swimming