The attackman¹s responsibility is to score goals and help his teammates score goals by passing the ball. The attackman generally restricts his play to the offensive end of the field. A good attackman demonstrates excellent stick work with both hands and has quick feet to maneuver around the goal. Each team has three attackmen on the field during play.
Midfield: The midfielder¹s responsibility is to cover the entire field, playing both offense and defense. The midfielder is a key to the transition game, and is often called upon to clear the ball from defense to offense. A good midfielder demonstrates good stick work including throwing, catching and scooping. Speed and stamina are essential. Each team has three midfielders on the field.
Defense: The defenseman¹s responsibility is to defend the goal. The defenseman generally restricts his play to the defensive end of the field. A good defenseman should be able to react quickly in game situations. Agility and aggressiveness are necessary, but great stick work is more essential to attack. Each team has three defensemen on the field.
Goal: The goalie's responsibility is to protect the goal and stop the opposing team from scoring. A good goalie also leads the defense by reading the situation and directing the defensemen to react. A good goalie should have excellent hand/eye coordination and a strong voice. Quickness, agility, confidence and the ability to concentrate are also essential. Each team has one goalie in the goal during play.
Above info from US Lacrosse.