R&B stands for "Rhythm and Blues". It originated in 1940s in the African-American music circles. It essentially takes blues and adds rhythm to it, and it is a fusion of blues, gospel, and soul music. Contemporary R&B is a a huge part of today's mainstream American music.
Throughout decades, the term R&B changed it's definition. In 1940's it was defined as urbane music with heavy insistent beat. In 1950's it referred to music incorporating electric blues, gospel, and soul. By the 1970s, it became a blanket term for soul and funk. In the 1980's a newer style of R&B developed, called "Contemporary R&B".
Commercial R&B bands from 1950s to 1970s, usually consisted of piano, one or two guitars, bass, drums, and saxophone. Performances were rehearsed to the point of effortlessness. While singers were emotionally engaged to the lyrics, they remained cool and in control. Music typically followed predictable patterns of chords and structure.