I often get that question. Or something in line with "so the pilot flies the plane, what do you do?"
That might seem like a weird question, but a lot of people outside the flying community seem to call the Captain "the pilot", and indeed he/she is, but there seem to be some confusion as to what to call the First Officer, let alone what we do. Maybe now is a good time to clarify that I am a First Officer myself...
"in a multi-pilot environment, we help each other out with everything to make the flying as smooth and safe as possible, it goes both ways."
Of course we are both pilots, the Captain in the left seat and the First Officer to the right. That is our working positions on board, the chain of command if you will. All aircraft need a Pilot in Command, or PIC. In the United States, First Officers are typically called co-pilots, possibly adding to the confusion since "co-" would suggest 'helping' the 'pilot' with his/her duties. Well, part of that concept is true of course. The First Officer acts as the Second in Command, or SIC. Should the Captain for some reason be incapacitated or relieved of duty for safety reasons, the First Officer assumes the role of Pilot in Command (but never the position as Captain). And in a multi-pilot environment, we help each other out with everything, to make the flying as smooth and safe as possible, it goes both ways.
The concept we use to optimize the synergy (best possible level of cooperation) in the cockpit is called Crew Resource Management (CRM). This is the main focus of Multi Crew Cooperation (MCC) Courses.
But we also assume other roles onboard. Let's go back to the question: "Who flies the plane?" Well, we both do! But we take turns.
The roles I talk about are called Pilot Flying (PF) and Pilot Monitoring (PM). Very briefly, the PF does all the flying; physically takes off and lands, hand flies the aircraft or operates and monitors the autopilot. The PM takes care of the other stuff; talking to ATC (Air Traffic Control), follows up time and fuel burn in the flight plan, assisting the PF by 'monitoring' the flight. That's the short version. The reality of course is a bit more complicated. The roles actually change during a single flight depending on if you are on the ground or not (you can indeed be PF on the ground).
Now you know what to answer next time you get the question of who the pilot is or who flies the plane. We are both pilots and we take turns flying the aircraft.
More in depth of what these roles mean and how they are connected to what is called the flight deck's "area of reponsibility".