If you didn't know Hip Hop is one of the few original american art forms/cultures. There are four elements to hip hop. Graffiti, break dancing, DJing and MCing (or rapping to the layperson) The garbage you may hear on the radio or see on tv most like has no connection to the hip hop culture. Do yourself a favor and listen to some of the folks that are trying to keep the real spirit of the culture alive, not the vampires that are sucking it dry to make a buck.
Good Artist to Start listening to:
Del tha Funkee Homosapien - Deltron 3030
Aesop Rock - Labor Days
Eyedea - First Born
History - http://en.wikipedia.org/wiki/Hip_hop_culture