IN the 1970s, a cultural movement sprang up out of dance music. Disc jockeys at clubs and block-party dances began to talk over the beat, using wit and rhyme to please the crowd. All but a very few of these DJs and masters-of-ceremonies (MCs) were African-American, as was true of their early audiences. They often described the problems of inner-city America, providing an alternative to saccharine soul and vacuous disco.These new musicians tried to describe reality. Sometimes it was poetry, and sometimes it was garbage. Now, if you walk into any music store, or pry the headphones off a teenager - whatever his or her race - you're likely to hear rap or hip-hop music. This week the Monitor talks with three rappers, different in style, sex, and race, about their music. * Today: Queen Latifah. * Tomorrow: Ice-T takes music to its abrasive limits as the godfather of hardcore rap. * Friday: The multiracial group 3rd Bass aims for rap that agitates for change but still makes people want to dance.