...as opposed to "swing eighths". Rock tends more towards straight eighths, i.e., an even pulse of eighth-notes in the rhythm. The perceived "bounce" of various styles of jazz (say, from the turn of the 20th century to the 60's) comes from a distortion of that pulse - every second eighth-note is slightly delayed. Not a hard-and-fast rule: e.g. a lot of early rock and roll is swing and rhythm and blues based; Chuck Berry sometimes superimposed both types of pulse.