Rhythm and blues is a term describing popular African American music that originated in the 1940s. The term was originally used by record companies to describe recordings marketed predominantly to urban African Americans, at a time when "urbane, rocking, jazz based music with a heavy, insistent beat" was becoming more popular.
The term has subsequently had a number of shifts in meaning. In the early 1950s and beyond, the term rhythm and blues was frequently applied to blues records. Starting in the 1950s, after this style of music contributed to the development of rock and roll, the term "R&B" became used to refer to music styles that developed from and incorporated electric blues, as well as gospel and soul music. By the 1970s, rhythm and blues was used as a blanket term for soul and funk. In the 1980s, a newer style of R&B developed, known as contemporary R&B.
For Rhythm & blues in its original context, see http://www.last.fm/tag/rhythm%20and%20blues