hmm, there's a lot of controversy to this.
there's some who believe that hip hop is a life style or way of living (4 elements) and has nothing to do with the current "rap" scene that you see on MTV (all the bling bling, 20 in rims, hoes/bitches, etc.) there's even the distinction of being just a rapper and being an MC, even though both do what is basically the same thing. it's all in the ethics. compare performers like Madvillian or Blackalicious to P. Diddy. I guess you can call both of them rappers/MCs, but they are very very different in terms of what they believe, their motivations for doing what they're doing, and so on.
i guess you can say it's sort of like the contention between the indie rock scene and bands such as Linkin Park. what's the difference between indie rock and radio rock? the same can be said for hip hop and rap.
The mainstream music scene has used the term hip hop for almost everything though, it recently seems. so those who don't follow the scene too closely, they essentially are the same thing.