Rock music is slang for rock and roll music. Originating is the 40's and early 50's, it is a genre of music that has evolved from an experiment between boogie woogie/blues(ex.John Lee Hooker), gospel(ex.Mahalia Jackson) , and country/western(ex.Jimmie Rodgers) along with swing and jazz music known as rockabilly into the hundreds of subcategories and counting that we know of today. The REAL definition is a colaboration between all genres of music of that time. A colaboration of all races, creeds, and positions in society that couldn't have happened if it weren't for the hundreds of things that went terribly wrong and completely right in that period of time. In short, rock music was a miracle of music.
A lot of the reason rock music was formed was when the swinging Big Bands of the 40's were split up by WW2, many either died out or became smaller. They had to rely more on vocals and guitar than the large instumental compositions that they had started out with. Buddy Holly's was the rock music band that set the standard of 3 guitars and drums.