buoy vs spar buoy

buoy and spar buoy both are nouns.

Word	Noun	Adjective	Verb	Adverb
buoy	Yes	No	Yes	No
spar buoy	Yes	No	No	No

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

buoy: a float attached by rope to the seabed to mark channels in a harbor or underwater hazards; typically brightly-coloured
spar buoy: a buoy resembling a vertical log

Other hyponyms of buoy include acoustic buoy, can, can buoy, conical buoy, nun, nun buoy.

buoy (noun)	spar buoy (noun)
a float attached by rope to the seabed to mark channels in a harbor or underwater hazards; typically brightly-coloured	a buoy resembling a vertical log