The 3-5-2 is a formation in soccer consisting of three defenders, five midfielders, and two strikers (and of course a goalie). The defense consists of two marking backs, who generally play a zone defense as opposed to man to man. It is their job to spot the two opposing strikers (forwards) and stay with them while the other team is on offense. When their team is on offense, the two backs are free to make runs up the side to flood the opposing defense, as long as a midfielder drops back to take over their position while they are gone. The third defender in this system is often a sweeper, who stays behind the two marking backs by about five to ten yards, and "sweeps" back to stop any players that break through the first lines of defense. If a defender is beaten and his man goes against the sweeper, the defender himself must fall behind the sweeper becoming the sweeper himself until the ball is removed from their defensive area, or until the sweeper can recover to his original position.
The midfield consists of two wings (the two outside midfielders), two defensive midfielders, and an offensive midfielder (or two offensive and one defensive midfielder). It is the job of the wings to make runs up and down the line to relieve pressure on the middle part of the field, and if possible to carry the ball down the line to cross the ball into the strikers. They will also often make crossover runs with the strikers where they go to the middle and the strikers go outward, to confuse the defense. Sometimes they must also mark (guard) the other team's wings themselves. The two defensive midfielders try to control the middle of the field, and play further back marking up the opposing midfielders, and intercepting free kicks (such as goal kicks, or offsides kicks). When playing with a single defensive midfielder, he usually plays as a "stopper" which is basically a sweeper for the midfield. The offensive midfielder(s) play further up than the defensive mids, and he/she usually make runs up the middle in support of the strikers, sometimes even switching places with them to confuse the defense. They also rush into the 18 yard box when crosses are made from the outside in search of a goal.
The two strikers are the stars of the team, and are usually the smallest and fastest players. They almost never come far back into their own half unless desperately needed, although they are still the first line of defense. Their job is to wait until their team wins control of the ball, and then make a run to the corner looking for a pass. (They do not run down the middle because then a pass over their head will go straight to the goal keeper). Then they will either dribble the ball in for a shot or pass it to another striker or midfielder who will try to score.
The 3-5-2 system creates a very strong midfield that is the backbone of the entire formation. If the midfield does not fall back, then the defense will often be over run. If the midfield does not push up on the attack, then the two strikers will be unable to score by themselves. It is vitally important in this system for the wings to make good runs, and for the offensive midfielder to support the strikers from behind. The defensive midfielders must win punts and free kicks in the center, or the defense will be overwhelmed, and few offensive opportunities will be created.