You hear it every flight, the introduction of the Captain and First Officer of your flight, but did you ever stop to ask, “What’s the rank all about?”
The captain of an airplane is what the FAA calls the Pilot-In-Command. They are the pilot responsible for everything that occurs onboard their airplane during flight. This includes all paperwork, safety-of-flight issues, customer service, and crew member issues. The captain is quite literally the final authority on everything that happens.
The first officer is directly responsible to the captain and is there to both assist the captain with his duties and to assist in flying the aircraft. Just like the captain, the first officer is a fully trained and certified pilot on that aircraft. He/She is responsible for any tasks delegated to them by the captain. These tasks usually involve pre-flight and post-flight inspections of the airplane, weight & balance and performance calculations, and running aircraft checklists. Unless rare circumstances exist, all pilots are hired by an airline as a first officer. It is the entry-level airline position and is designed as an experience builder.
For most airlines it takes about 6-10 years for a first officer to accrue enough seniority within the company to upgrade from first officer to captain. By that time the first officer has acquired enough flight time and experience in that aircraft to be competent and safe a captain.