Graffiti is considered one of the four elements of hip hop, along with emceeing (rapping), DJing, and b-boying (breakdancing). Graffiti, like the other three elements, is an artform, a means of cultural expression. Like the other forms of hip hip, it also expresses resistance. Graffiti challenges, for example, mainstream notions of what counts as art, what counts as public space, and what counts as property, just as emceeing/DJing challenges what counts as music, and bboying challenges what counts as dance.
Graffiti, unlike the other forms of hip hop, is more easily misunderstood because it is often done illegally and the artists are often secretive about their real identities. As such, it has not received the kind of positive mainstream recognition afforded to rappers, DJ's, and dancers. Some professionals in the art world have embraced graffiti, but that doesn't mean that the public at large understands and appreciates what graffiti is all about.