The Oud, a Middle Eastern instrument, is a plucked lute chordophone. Five strings run down a fretted, pear-shaped body from five uniform tuning pegs. The body is made from woods like maple, rosewood, mahogany and walnut. It originally had four strings, but between the 8th and 10th centuries, a famous musician named Zitiab added the fifth string. It is plucked by a plectrum (basically a really large guitar pick) which is often made from a trimmed eagle feather. A deep and mellow timbres are created by playing it. The neck bends back about 30 degrees giving it an organic, twisting appearance. It greatly resembles the Chinese Pipa, a direct descendant of Middle Eastern lutes.
The Oud is arguably one of the most influential instruments in the history of music. Not only did Chinese and Japanese acculturate it for their musical needs, but also nearly every civilization on the continent we call Europe. Even the English word lute, which is used by ethnomusicologists to describe any guitar like stringed instrument, comes from Oud. Al-'ud, meaning "branch of the wood," was changed to laud for the Spanish which in turn became the lute for the English. This seemingly simple transformation of the instrument took place through the centuries. It could be argued that the Oud is a great ancestor of the electric guitars we hear, love and smash today.
They are still played in the Middle East and are considered by some to be "The king of all instruments." Singers that wish to accompany themselves, Middle Eastern composers, ensembles wishing to preserve traditional music and students studying ethnomusicology are the biggest users of the Oud. In recent times, electric Ouds are actually made in addition to the older traditional style.
'Ud is an alternative spelling. 'Idan is the plural form.
Sources:
World music class
http://almashriq.hiof.no/lebanon/700/780/fairuz/legend/instruments.html#ud
http://www.oud.net/
http://www.music.ucsb.edu/mee/instruments.html