It seems safe to say that we will see a split (I think) in the rap genre, where you get the Kanye Wests and Commons away from Soulja Boy and T-Pain. Its a different kind of rap that makes very different kinds of money, and as they say, a divided house cannot stand.
There's been a split in rap music in rap music since about 1986 (by my estimation); there isn't really a commercially-defined name for the different types of it though. Rap music was borne out basic poetic structure, and the music behind the lyrics were based on backgrounds of pop/R&B standards of the time. Electronic instruments and turntables helped create the sound; it had to be portable since many DJs/rappers appears at parties and outdoors in the early days. There were no moving vans, and no certainly no tours, just gigs based on flyers printed up for contests.
Electronic instruments were changing too, both in complexity and technology, so there were splits in dance-oriented rap (the basis of House, when combined with Detroit Techno, which influenced Elecktrofunk before House) and the lyrical stuff. Later came different rap content; sub-genres were based on that, conscious lyrics intended to raise awareness and lyrics intended to talk about basic everyday things and feelings. Rap was an underground music genre at that time; many rappers and many DJs, but only a handful were signed to record labels at the time.
Sometime around 1986 when Run-DMC remade Aerosmith's "Walk This Way", you had rap music reaching out to a different audience, it became accessible as a crossover hit to various radio stations. There were a few (semi-)independent record labels (Tommy Boy, Cold Chillin', Def Jam), but nothing like we see today. A few years later, rap became bigger and bigger, more airplay, more radio stations, and videos. That meant the major labels could open their checkbooks and sign whatever talent was out there.
Rappers today
still argue until the cows come home over who's a sell-out and who's underground, and that's been argued for ages; it's a two-way street, since the underground can't make money and garner much interest unless there's pop-act giving exposure to the genre, and vice-versa...if nobody's making anything creative out of the genre (i.e. not selling-out), it dies out from over-exposure. You could probably substitute "rappers" with any other genre or musician and likely get the same result.
You have hardcore, gangster (gangsta), trip-hop (influenced by early.mid-90s electronic music), and experimental and even obvious stuff that defies classification. (Where do you put the Beastie Boys, for example?) So there are different (sub-)genres of rap music; but like any other basic genre, it's hard to classify them, and you can get a headache just thinking about the labels.
