Nowadays "Rap Music" is synonymous with "Hip Hop". But Hip-Hop is more than just that, there is beatboxing, DJing, breakdancing/B-boying and grafitti.
A Hip Hop track can just be an instrumental that track was even coined "Hip Hop's national anthem" by the Afrika Bambaataa and DJ Kool Herc.
Now, the reason why Rap Music now means the same thing is because, Rap is the most outspoken of the elements. Music is sound so the visual elements like breakdancing and graffiti aren't going to be on a record, they might be an image of on the album cover but the sound is the most important part. Not all hip hop tracks have beatboxing, and just like a lot of music, there is usually an original composed piece, then again, a lot of hip hop tracks have sampling, DJing is a lot more outspoken than the other elements but not as much as rapping, the human vocal touch.
When I first heard The Message by Grandmaster Flash and the Furious Five, I thought the rapper was Grandmaster Flash, but the rapper was Grandmaster Melle Mel, Grandmaster Flash was the group's DJ, because in its early days, the DJ was the most important part, Rapping/MCing itself evolved from DJ's talking over the track telling people when their next gig is going to be and everything.
http://hiphop.wikia.com already redirects here, but should the name of this wiki be changed the "Hip Hop wiki"? In my opinion yes.