well according to encarta hip hop is:
popular music, art, and dance: a form of popular culture that started in African American inner-city areas, characterized by rap music, graffiti art, and breakdancing
and rap is:
popular music with spoken rhythmic vocals: a vocal style in which performers use rhythm and rhyme to speak in verse over music with a strong beat, usually hip hop. Rap developed from African American hip hop music and culture in the 1970s.
I personally say that "hip-hop" is what they called it after "rap" sold out. but that's just me.