It may help to familiarize yourself with some of the history and various styles of rap music.
Old School Rap started in the late 70s /80s with groups like The Sugarhill Gang and Grandmaster Flash. Some of their classics can still be heard on the dance floor however you may find their sound a bit primitive these days.
Rap hit a sort of Golden Age in the 80s with acts like Run-DMC, LL Cool J, Public Enemy, KRS-0ne, Erik B & Rakim, Rob Base, Slick Rick and so on. The music from this period was kind of a mix of afrocentric inner city politicicism and party music.
With acts like NWA and Ice-T, Gangsta Rap (specifically West Coast) was born. NWA consisted of Dr Dre, Ice Cube, Easy E and a couple of other guys. It didn’t become commercially viable as a music genre, however, until 1992, when Dr Dre’s The Chronic was released and later his protegee Snoop Dogg’s Dogiestyle. By then, of course, NWA had broken up and Dre and Easy E had their falling out.
About the same time on the East Coast, a group of rappers got together and called themselves the Wu-Tang clan. While not initially as mainstream as Dre or Snoop, they continue to be a favorite of white suburban kids everywhere.
In addition to the gangster rap of the day (both East and West Coast), you also have the smooth sound of various non-gagsta hip hop acts like A Tribe Called Quest, The Fugees, Brand Nubians, Arrested Development and so on.
You also have the “New Jack Swing” sound of the early 90s that mostly consists of hip-hop influenced R&B like Color Me Badd, Bell Biv Devoe or Boys 2 Men.
In the mid 90s, gangsta rap started to take on a more grandious image with acts like Notorious BIG, Tupac (both dead) and Puff Daddy (now Diddy). Rap started to become more about the bling and less about the street.
Southern rap has also started to become popular with acts like Ludacris, T.I., Outkast, Witchdoctor, Young Jeezy, Yung Joc, The Ying Yang Twins, and Lil Jon (see Crunk).
And several Midwestern artists like Eminem and Kanye West have risen in popularity over the years.