pop music vs popular music

pop music and popular music both are nouns.

Word	Noun	Adjective	Verb	Adverb
pop music	Yes	No	No	No
popular music	Yes	No	No	No

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

pop music: music of general appeal to teenagers; a bland watered-down version of rock'n'roll with more rhythm and harmony and an emphasis on romantic love
popular music: any genre of music having wide appeal (but usually only for a short time)

Other hypernyms of pop music include popular music genre.

pop music (noun)	popular music (noun)
music of general appeal to teenagers; a bland watered-down version of rock'n'roll with more rhythm and harmony and an emphasis on romantic love	any genre of music having wide appeal (but usually only for a short time)