Well, it's more of a timeline really: There's the oldschool Hip-Hop, like Grandmaster Flash and such, then there was east and west coast rap, east coast was kinda silly, and stuff, while west coast rap was harder and grittier, stuff like NWA and such, There's "Progressive Hop" as Ishkur calls it (AKA Underground or Alternative hip-hop) and that's stuff like the Black Eyed Peas, Del, the Funkee Homosapien, there's Abstract/Experimental hip-hop, such as DJ Food and Coldcut, there's G-Funk, which was created by Dr. Dre, and is west coast gangsta rap, and now there's full on Gangsta rap, which is pretty much the entire rap industry now.
It may not be like metal where genres are more like adjectives, and can be added and subtracted and used as influences, but are more like time periods, and mentalities. The east coast/west coast stuff was going on in around the mid to late 80's and early 90's, then came g-funk, which became wildly popular for about 5 years in the mid 90's, and after that, Gangsta rap took hold and hasn't dropped out yet (though it seems to me like everyone's finally getting to the point where they don't care about what 50-cent is doing. So maybe in another couple years or so we'll see more alternative stuff breaking through. The Black Eyed Peas certainly paved the road for more Alternative Hip-hop to break through, and the Gorillaz have always had a good following, so who knows, maybe the 2010's will be dominated by Alternative/Progressive Hip-hop.)
