"Rhythm and Blues" is a term for a musical style. It is sometimes called "R&B" for short. As mkb points out,
the term arose because the music features a rhythm section supporting a blues-based melody. Nowadays, term has been debased
so as to become almost useless by its application to a large number of very different genres which are only vaguely
related.
There seem to be several categories of use.
- 1940's African-American music; all of it except Jazz and Gospel, really. The term R&B was used
as an alternative to "Race Music". It included pure Blues, Boogie-woogie and ballad crooners like the Ink
Spots. If it wasn't performed by whites, and you didn't know what else to call it, you called it R&B. I think this is how the term "Urban" is abused today.
- The Stax, Motown or Philly sound; Al Green, Otis Redding, Marvin Gaye, The Dells and
the like. The term was now applied to this narrower genre.
- British Rock of the 60s; Including The Who, The Rolling Stones and The Kinks. (The Who have a boxed set
called "30 Years of Maximum R&B"). There is little similarity between the rock sound of this genre and the sounds of
the above genres- yet they have an identical name.
- Big Band Boogie-Woogie- like Jools Holland's Rhythm and Blues Orchestra. This seems to be an isolated
one-off, with no connection to other R&B forms.
- Contemporary popular forms; Destiny's Child, Mary J. Blige et al. Aficionados of all the above forms
hate this usage.
Whilst looking through the Internet for the above info, I spotted the terms "Rock R&B" and "Soul R&B" in the
google cache. The site that used them no longer does. I think that these terms are more descriptive and should be
preferred, especially when discussing the history of popular music.