The attackmen (in general) are the team's goal scorers. To play attack lacrosse positions you must be quick and agile, with great stick skills and the ability to score a goal on command. Attackmen play close to the whole game. This is because they only exert themselves when they are on offense and don't have to run back and play defense.
Middies are the heart of a good lacrosse team. They are constantly running, and therefore have to substitute more often. From taking face-offs, to scoring goals, to playing defense, mid-fielders do it all.
A good defense is vital to a lacrosse game. If you want to be an enforcer, then this is the position for you. Defense-men play with longer shafts (usually 60 inches), so they can beat on the opposing team easier. Being quick on your feet is a must for this position. Good vision is also vital, because you must always know where your guy is, as well as where the ball is.
The goalie. Much like every other sport, the goalie is the single most important of the lacrosse positions on the field. A "short" memory is required to be a lacrosse goalie, because you will almost always have 5-10 goals scored against you in a game.