heres allmusic guide's breakdown of HIP HOP:
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In the terminology of rap music, Hip-Hop usually refers to the culture — graffiti-spraying, breakdancing, and turntablism in addition to rapping itself — surrounding the music. As a style however, hip-hop refers to music created with those values in mind. Once rap had been around long enough to actually have a history, hip-hop groups began looking back to old-school figures including MCs like Kurtis Blow and Whodini, and DJs like Grandmaster Flash and Afrika Bambaataa. In fact, the latter's Zulu Nation collective sprang up in the late '80s around two of the most notable hip-hop artists, De La Soul and A Tribe Called Quest. With rap music's mainstream breakout during the '90s, dozens of hip-hop artists pointed the way back to the old school, including underground rappers like Mos Def and Pharoahe Monch.
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so there it is... I guess that sort of right... Hip Hop doesnt explain the culture of the whole thing while rap refers strictly to the music... hip hop doesnt have to have rapping within it... instrumental beats can be hip hop... breakdancing is considered hip hop, all that stuff they said... but you can't rap without vocals...