rock'n'roll vs popular music

rock'n'roll and popular music both are nouns.

Word	Noun	Adjective	Verb	Adverb
rock'n'roll	Yes	No	No	No
popular music	Yes	No	No	No

As nouns, popular music is a hypernym of rock'n'roll; that is, popular music is a word with a broader meaning than rock'n'roll:

rock'n'roll: a genre of popular music originating in the 1950s; a blend of black rhythm-and-blues with white country-and-western
popular music: any genre of music having wide appeal (but usually only for a short time)

Other hypernyms of rock'n'roll include popular music genre.

rock'n'roll (noun)	popular music (noun)
a genre of popular music originating in the 1950s; a blend of black rhythm-and-blues with white country-and-western	any genre of music having wide appeal (but usually only for a short time)