jayjamjah is right, hip-hop is a lifestyle that includes: b-boying (breakdancing), MC-ing, and DJ-ing. It's the MC's that rap, but not everyone that raps is considered an MC by "true hiip-hoppers". The original purpose of an MC would be to add a humorous comments over a beat during block parties, etc... But then it developed into something bigger where now many say it is the job of an MC to educate and send a positive message rather than talk about hoes and cars and gold chains. Here's a decent video, I hope was helpful.
YouTube - "The Definition of Hip Hop" - Directed by Konee Rok