Rock and roll sprang from a combination of African-American genres, Western swing, and country music that exploded in post World War II America. Rock and roll sprang from a combination of African-American genres, Western swing, and country music that exploded in post World War II America. Jim O'Connor explains what constitutes rock music, follows its history and sub-genres through famous musicians and groups, and shows how rock became so much more than just a style of music influencing fashion, language, and lifestyle.