1st Gen-The Party Rocker Era (1974-1978)-DJ Hollywood, King Tim III, Lovebug Starski & other cats started hip hop in the parks in this era. Rap was more about catch phrases than lyrics then. Breakbeats, and the DJ's that played them, were the background to this era.
2nd Gen-The Electro Era(1978-1980)-Cats like Mantronix, Newcleus & Jam Pony Express started getting into electronic instrumentation & using that to define their music. Rappers began to move past the catchphrase style to lyrics. Beats became stotic, as the TR-808 had a hell of a time replicating a true drum pattern.
3rd Gen: The B-Boy Era(1980-1987): This is the era that rap as we know it truly began. The Run-DMC & LL Cool J style dominated, with lyrics becoming more complex but still a callback to the catchphrase style. The music became more detailed, with the thudding TR-808 beats backed by the EnSonic keyboards & samples galore.
4th Gen: The Bragging Era (1987-1989): This era is where the Kane, Kool G. Rap & Rakim come in. The DJ ceases to be the center of the party & the MC is now dominant. Lyrics become extremely complex, with multis, metaphors & details that rival the deepest poems & stories. The music doesn't change too much, however, with samples of James Brown's records creating the soundscape of many, if not all, of these songs.
5th Gen: The Alternative Hip-Hop Era(1989-1996): The lyrics & influence of groups such as the NOI & the Zulu Nation, along with a growing consciousness of Afrikan culture due to media exposure creates this era. Cats like KMD, Poor Righetous Teachers, X-Clan, BDP & others push the lyrics to the reality of ghetto life & the blackness of hip hop itself. Samples from jazz, disco, spoken words & other genres began to show up, along with the art of chopping up a song (mastered by DJs like Premier & Evil Dee). As the sampler's memory grows, so does the backdrop of music that can be painted in this era.
6th Gen: The Gangsta Era (1994-1998): In the midst of the Alternative Hip-Hop Era, the West Coast moved forward, taking the funk of George Clinton & the Bragging Era & created their own version of hip-hop, based on their enviroment. First called Reality Rap, once NWA set the fire, the blaze created a wave of gangstas that met & surpassed the
Alternative Hip-Hop Era. Rappers like Snoop Dogg, Warren G., Ice Cube & others slowed down the flow but still kept the rhymes complex while telling their stories.
7th Gen: The Hip-Pop Era (1998-2001): The popularity of Gangsta rap, due to George Clinton's funk (IMHO), created an economic boom. The economic boom had a side effect, however....now, everyone & their momma wanted to be rappers. The marketplace supported it but in return, the lyrics were either simplified or exaggerated for the appetite of a larger audience. This lead to artists merging their form of music with rap, such as rap-rock icons Limp Bizkit & Korn, trip-hop artists like Portishead, pop-lite acts with bars like LFO & LEN or straight alternative like Gorillaz. The authentic rappers were still around (see WC, MC Eiht) but either chose to ride the wave to riches (Coolio), keep it authentic & stay typecasted (MC Ren) or change their styles to something a little more lyrical to stand out (Ras Kass, Ahmad, Souls of Mischief, The Pharcyde).
8th Gen: The Southern Era (2000-2005): In the wake of the Hip-Pop Era, the South stood quiet, building on the funk of the west & the lyricism of the East, mashing it up into a hybrid that started the Southern Era. Held together by country bonds & backed by a independent hustle that guaranteed wealth before even sniffing the mainstream, cats like No Limit, Cash Money, Suave House & the Texas movement freed them from the compromise of a record label...which they were now able to meet on equal terms with their own style. UGK, Master P, Scarface, Vell Barkardy, Mike Jones & more all put out music that sold, due to not being shoeboxed in & because it was their reality rap with just a little bit of gangsta in it.
9th Gen: The Money Era (2005-2009): Samples have always been a major part of hip-hop & rap. However, when producers decided to push the envelope of sampling from just a piece of G-Funk to an entire song's backing, you get a popular hit re purposed for the now. Puffy & Jermaine Dupri jumped on this bandwagon, seeing the hole that the West & South left open & converting it for East Coast ears by having lyricists spit the hardest of bars against the music's bouncy flows. Taking the offbeat Southern cadence, along with the reality raps that the West Coast perfected, artists such as Biggie Smalls, Craig Mack, Bow Wow, Da Brat, Nelly, Nas, T.I. & others followed this blueprint to fame & fortune.
10th Gen: The Trap Era (2010-2014): Music is universal, as is drug use. When the drug of a music culture crosses over to another, it completely alters the entire format of the genre. Rap music, by this time, had diversified itself amongst all the different genres. To be honest, every genre had a rap variant to it. However, the Money Era's lust for nepotism, along with the Southern Era's support base consisting of strip clubs & hole in the walls, created an opening for a new drug to take way: lean. Lean slows everything down....due to that, the music sloooooooowed down & the lyrics followed along. Molly followed soon after, coming from the grime/rave/trip-hop subculture that ran parallel with hip-hop ascendancy. Beats became slow & plodding, lyrics became simpler yet more direct & a number of variants was formed from local cities pushing their specific culture into the music (drill music, mumble rap). Artists like Future, Chief Keef, Wacka Flocka Flame & others define this era.
11th Gen: Now (nothing sticks out yet) (2014- ): So far, it seems that the music of the Trap Era is waking up & retaking lyricism. Beats are still slow & plodding, but the instrumentation behind them isn't. Cats like Migos & Rae Sremurred are the standards, so far.
Please note a few things:
A lot of these eras blend into one another. I'm aware of that.
Also, I am aware that not everyone is doing the predominant music of the era. The era is defined by the artists that are the rule, not the exception. So, even though Kendrick Lamar is dominant in the Trap Era, he is not the exception. Same thing goes for J. Cole in the Trap Era, etc.