A field general refers to any player in any sport that, when on the field, acts as a leader for his team. He's the one that others turn to for direction, and often he is the one either calling plays or relaying them from the coaches.
In football, this player tends to be the quarterback on offense and the middle linebacker on defense; both are generally regarded as the most cerebral on their side of the ball.
In basketball, the term is "floor general", and it is almost always the point guard. They are the ones calling plays and directing traffic in transition.
Examples of field generals in football include Tom Brady, Peyton Manning, and Ray Lewis.
Examples of floor generals include Isiah Thomas, and more recently Derek Fisher and Chris Paul.