The rababa was first mentioned in the 10th century, and is the earliest known bowed instrument. It became prominent in medieval Arabic music, and is the essential instrument in Arabic classical music, along with the oud, the nay , and various percussion instruments. It was mentioned in al-Jahiz’ “Collection of Letters”.

The rababa comes in various regional shapes, quadrilateral, pear-shaped and boat-shaped, and can have one, two, three or four strings.

The Bedouin rababa is probably the most important instrument employed throughout the Arab region. It has a quadrilateral sound box covered with skin and a single string made from horsehair, and is played with a horsehair bow. It was traditionally played by the poet-singer to accompany heroic and love songs. On the Mediterranean coast, the waisted wooden rababa accompanies improvisational poetic contests (zajal).

Its variant in Iraq is known as the Joza, indicating the coconut shell from which it is made.

In Morocco the rababa is played almost exclusively by professional musicians, usually from hereditary lineages. The instrument plays a dominant role in Andalusian music, and is also used by street musicians. It is typically used for voice accompaniment by the rwayyis, or professional entertainers.

The boat-shaped variety was introduced by the Arabs to Spain in the 11th century.

References:
The New Grove Dictionary of Music and Musicians. Second edition, 2001. v 17, p 136.
al-hakawati http://al-hakawati.net/Art/ArtDetails/53/الرباب