Gangsta rap is any rap song which depicts the violence and crime of street life. This depiction can be from the standpoint of the
gangster or some other party. Some gangsta rap glamorized street life, whereas other examples decry the destruction.
Gangsta rap, while made famous by West Coast acts such as NWA and Ice T, was probably first introduced to the public by none other than Boogie Down Productions on their first album, Criminal Minded. This album featured the song 9mm Goes Bang in 1987. This, to my knowledge was the first gangsta rap song put out on wax. The style did not resonate with the hip hop crowd (not all rap is hip hop and not all hip hop is rap), and was reinvented for California consumption the subsequent year by NWA. It then sprouted in Houston, Texas with the Geto Boys, most notably Scarface, and has since spread around the country, including back to its point of origin, New York City.