Rap music is another evolution of American music. Like all true American music, it originates with African Americans, follows little to none of the "classic" rules of European music, and pays no homage to the majority culture. Rap music IS was rock n' roll WAS, the music of rebellion. It has been adopted as one of the musics of choice by poor city dwellers in almost every urban center of the Western World. Ironically, rock n' roll was a previous expression of African American music, later accepted and adopted (or co-opted) by the mainstream culture. The same may yet occur with rap, at which point poor African Americans will develop some other music form.
Jamaican born DJ Kool Herc moved to New York in the late 1960's and brought with him the Jamaican tradition of "toasting", which involved reciting improvised rhymes over instrumental sections of reggae records. He used twin turntables and cut back and forth between two seperate records to create a new sound. He would chant things like "Throw your hands in the air/And wave'em like ya just don't care!". This type of crowd pleaser was known as "rapping" to the crowd. At that time rap was not yet known as 'rap' but called 'MCing'. He later turned his attention to DJing and let two friends Coke La Rock and (the orginial) Clark Kent handle the mic. This was rap music's first MC team. They became known as Kool Herc and the Herculoids.
Those who say it's not music, or that it's "ghetto music", have no connection with the history of THEIR own rock music - making them cultural bastards.