A Cabin Boy is hired on a ship as part of the crew. Their main duties tend to be the unimportant jobs, such as cleaning or running errands for the captain, and there is often more than one aboard a ship. Cabin boys often aspire to become sailors themselves.
The first cabin boy introduced was Coby, who, by accident, became the reluctant crew member of Alvida. After being freed by Luffy, he later became a willing cabin-boy of the Marines, alongside a reluctant Helmeppo, before they were both taken in by Garp and rose in rank.
Other cabin boys mentioned are Buggy and Shanks who served on board the Oro Jackson, under captain Gol D. Roger, though they were considered more to be apprentice pirates than cabin boys. Luffy was a cabin boy on Baratie and left after his fight with Don Krieg (although it was a punishment from Zeff for damaging a portion of the Baratie).