A goalie does what a goalie does in other sports. It guards the net and tries to block the shot from the opposing team.
Henning Schlottmann (User:H-stt) / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)
A defender gets a longer pole than any other position and has to do many things. A defender has to clear the ball(Get it up the field), defend, listen to the coach the best, and stay aggresive, but not too aggresive that you get fouled. A defender has to listn to coach the best because they have the most callouts, for example, fall back, get to the hub, man down, pressur, and much more.
Bill Brine / CC BY (https://creativecommons.org/licenses/by/2.0)
The midfielder does the most running. They run back and forth between playing as defense or offense. They have to clear like a defender, and ride like an atackmen(You'll learn about riding soon). Also, a midfielder is the quarter back of the offense. They bring the ball up, and start the offense from there. They usually get the most assists because they start from the top and pass it to someone cutting in(Usually).
Tommy Gilligan, USMC / CC BY (https://creativecommons.org/licenses/by/2.0)
The faceoff midfielder tries to get the ball at the start of the possesion. They try to body the other face off midfielder to get the ball. The regular midfielders come in from the sides to help get the ball.
popofatticus / CC BY (https://creativecommons.org/licenses/by/2.0)
These players play midfield, but they have a defender pole. They do what a regular midfielder does, just with a longer pole to help on defense.
Sparky646 / CC BY-SA https://en.wikipedia.org/wiki/Kyle_Sweeney#/media/File:Kyle_Sweeney_playing_for_New_York_Lizards.jpg
The attack players play at X(behind the goal), or on the sides of the goal. They have to ride the, ball swing it, and score. Riding is when you try to sop the other team from clearing th ball. You do this to keep possesion for as long as possible.
Daniel Steger / CC BY-SA (https://creativecommons.org/licenses/by-sa/2.5)
That's all the positions in lacrosse.