Jay-Z

[Rap] is built around two kinds of rhythm.  The first kind of rhythm is the meter.  In poetry, the meter is abstract, but in rap, the meter is something you literally hear: it's the beat.  The beat in a song never stops, it never varies. . . . It's like time itself, ticking off relentlessly . . . .

But the beat is only one half of a rap song's rhythm.  The other is the flow.  When a rapper jumps on a beat, he adds his own rhythm.  Sometimes you stay in the pocket of the beat and just let the rhymes land on the square so that the beat and the flow become one.  But sometimes the flow chops up the beat, breaks the beat into smaller units, forces in multiple syllables and repeated sounds and internal rhymes . . . . The flow isn't like time, it's like life.

Another excerpt from his 2010 book, Decoded.