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.
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. On the other hand, should a ship lose all of its crew to the radiation, it will become a ghost ship and can only be captured by another ship with 20 or more crew.