Seafaring is a tradition that encompasses a variety of professions and ranks. Each of these roles carries unique responsibilities that are integral to the successful operation of a seafaring vessel. A ship's crew can generally be divided into four main categories: the deck department, the engineering department, the steward's department, and other. The reasoning behind this is that a ship's bridge, filled with sophisticated navigational equipment, requires skills differing from those used on deck operations – such as berthing, cargo and/or military devices – which in turn requires skills different from those used in a ship's engine room and propulsion, and so on.

A licensed mariner is a sailor who holds a license from a maritime authority to hold senior officer-level positions aboard ships, boats, and similar vessels. Qualification standards for licensed mariners are universally set by the STCW Convention adopted and promulgated by the International Maritime Organization (IMO), while the licenses of individual sailors are issued by the delegated maritime authorities of the member states of the IMO; these may vary in the details of the implementation, including the government agency responsible for licensing and the local names of the grades and qualifications in each particular country.