Here's a brief guide
The captain is pretty recognizable, so I'll skip over that one.
The first mate (often combined with the quartermaster which is why they're lumped together here) was the captain's right hand and kind of the vice captain. If the captain died or was unable to act as captain, the first make would run the ship. He made sure the captain's orders were followed and the other crewmembers did what they were supposed to. He also inflicted any punishments that needed to be carried out.
The navigator (who's also called the sailing master) was in charge of tracking the ship's route, keeping it on course, and making sure everyone arrived at the desired destination. They kept track of all navigation equipment and were actually extremely valuable if they were good because of how crucial they were to getting the crew to whatever they wanted to steal.
The gunner (also called a master gunner, a gun captain, or a cannoneer) was the person in charge of all canons, guns, and ammunition on board the ship. They lead the crew of men working the canons and were relied upon to make sure they hit their targets accurately and reliably. Also a very important position on any pirate ship.