Before the 1960s America was a country of racial segregation,especcially in the south.
Normally Afro-Americans and White Americans did not mix socially.
Each group had its own music and its own radio stations.
In the Fifties something strange happened:several young white American boys started listening to Rhythm and Blues music on Afro-American radio stations.They also liked the style of singing and tried to imitate the singers.One day in June 1954 one of these boys went to a small recording studio in Memphis,Tennessee and made a record.When people heard the record they thought the singer was an Afro-American.It was an immediate success and a new style of music was born.It was a mix of Rhythm and Blues and Country and Western.
The name of the singer was Elwis Presley,and he became the moust popular and influential singer of his generation.
He was the first person to give all American teenagers,both white and black,their own musicall identity.
So Rock and roll evolved in the United States during the late 1940s and early 1950s, primarily from a combination of the blues, country music, jazz, and gospel music. .
In the earliest rock and roll styles of the late 1940s and early 1950s, either the piano or saxophone was often the lead instrument, but these were generally replaced or supplemented by guitar in the middle to late 1950s. Classic rock and roll is usually played with one or two electric guitars (one lead, one rhythm), a string bass or (after the mid-1950s) an electric bass guitar, and a drum kit.
Rock and roll began achieving wide popularity in the 1960s. The massive popularity and eventual worldwide view of rock and roll gave it a widespread social impact.
Far beyond simply a musical style, rock and roll, as seen in movies and on television, influenced lifestyles, fashion, attitudes, and language. It went on to spawn various sub-genres, often without the initially characteristic backbeat, that are now more commonly called simply "rock music" or "rock."