A shanty (sometimes called a sea shanty) is a particular form of folk song – a song for soloist and chorus intended to be sung on board ship. It is a work song, having the same purpose as many of the songs sung by slaves in the plantations or in other situations where music not only can cheer the singers and lessen the boredom but assist in the rhythmic efficiency of the group.
It is not ancient. The first references to the form date only from the sixteenth century and the term itself – in English at least - is from the mid-nineteenth century.
There was usually a “shanty-man” selected to sing the solo which would begin each verse of a strophic song before being joined for the chorus by the other workers. A good shanty-man could cheer up the gang no-end by making up words as he went along, commenting on the ship, one or other of the officers, the weather or were obscene; but the chorus was always the same for each verse. (“A rovin’, a rovin’, for rovin’s been my ru-i-en, I’ll go no more a rovin’ with you fair maid” is a well-known example of the cast-iron chorus that would follow words that were not necessarily “printable”.) At times the words could be a safe and informal way of informing the captain that the crew had a gripe about something on board.
“What shall we do with a drunken sailor”, “Blow the man down”, and “The Rio Grande”, (http://www.youtube.com/watch?v=GyeFMNNZmsc) are well known examples. "Oh Shenandoah" is an example where the slower tempo can indicate the use of oars or poles during its use.
“The Grove Concise Dictionary of Music”
“Oxford Companion to Music”