Crew are the members of a starship. You need crew in order to pilot starships.
Whenever a ship is built, a full compliment of crew is automatically supplied to the ship. Crew can be killed through combat, ion storms (though it can't kill all crew), or even star cluster radiation. Should a ship lose all of its crew to radiation from Star Clusters, it will become a ghost ship.
If a ship loses all of its crew during combat, it is captured, and the new owner gets to command the ship with 10 free crew even if the hull normally has less than 10 crew.