It's simply a shortened version of 'rock and roll' which became a catch-all label for popular music, to distinguish it from black music, known under the umbrella title of 'soul'. This was back in the days when music was nice and straightforward, without several dozen sub-cultures. Now 'rock and roll' means either strictly 50's 60's rock and roll styles - Elvis / Buddy Holly / gene Vincent etc., or cross-over hard-edged rhythm and blues sounds, such as The Stones. Complicated isn't it!