art rock vs rock music

art rock and rock music both are nouns.

Word	Noun	Adjective	Verb	Adverb
art rock	Yes	No	No	No
rock music	Yes	No	No	No

As nouns, rock music is a hypernym of art rock; that is, rock music is a word with a broader meaning than art rock:

art rock: a style of rock music that emerged in the 1970s; associated with attempts to combine rock with jazz and other forms; intended for listening and not dancing
rock music: a genre of popular music originating in the 1950s; a blend of black rhythm-and-blues with white country-and-western

Other hypernyms of art rock include rock-and-roll, rock'n'roll, rock 'n' roll, rock, rock and roll.

art rock (noun)	rock music (noun)
a style of rock music that emerged in the 1970s; associated with attempts to combine rock with jazz and other forms; intended for listening and not dancing	a genre of popular music originating in the 1950s; a blend of black rhythm-and-blues with white country-and-western