Sweeper

The "libero" position was popularized thanks to German legend Franz Beckenbauer in the 1970's. At the 1990 World Cup, in Italy, even the Brazilian squad employed it for the very first time. Outraged Pelé severely criticized the national head coach Lazaroni as Brazil was eliminated early in the tournament. Conversely, at the World Cup 1990, Germany finished first while playing with a libero. Since Italy 1990, many teams have abandoned the use of the libero excluding many German and youth clubs. It's now evident that some teams are just not suited to play with a sweeper.

The sweeper must always be "the last player in defense". His job is to close down gaps left by other defenders. The libero roams laterally, a few meters behind the last line of fullbacks (although he tries not to sway too far towards the flanks). He analyzes the development of plays, anticipates where open angles and passing lanes are forming and make quick decisions about dealing with them. At times, more than one issue will present itself simultaneously thus making the sweeper's job even harder. Because they never mark enemy attackers, sweepers can move
a great deal forward when their team is in possession. This frequently finds the enemy defense unprepared. When an unmarked player is unexpectedly introduced to a play, the opposite defense is faced with tough choices. Overall, the sweeper should be very perceptive (usually experienced) with good ball handling skills and confidence.

Stopper / Center Fullback

The stopper is the center player in a defensive line. Some teams may use two stoppers in conjunction. Their main task is to mark the most advanced enemy forwards and fight with them in 50/50 encounters.

This is the most defensively-oriented field position in soccer. Stoppers have to remain in the backline, almost at all times and usually cover the shortest distance in a match (second only to goalkeepers). Stopper rarely get involved in offense, except possibly for set pieces like corners and during initial buildup.

Fullback

Fullbacks are the defenders positioned on each side of the stopper. They cover the space along the entire flank, therefore must be fairly quick. In defense, the fullback usually marks an enemy forward or covers his flank, awaiting incoming enemy wingers. Fullbacks get actively involved in their team's offense by staying wide, making overlapping runs and pushing up the flank. In modern football, it is not uncommon for a free (not marking) fullback to move up the wing and take the position of a winger thus spreading the enemy defense apart. To do this job, fullbacks need to have good speed and stamina.

Winger / Outside Midfielder

Even although the classical winger (like Garrincha) who dribbles up the sideline is nowadays being replaced by an all-purpose midfielder, the "old" winger position is still present in many teams today. The winger should be very fit as he's supposed to be active in both defense and offense. In defense, the winger must pick up the widest enemy player on his side. If he finds himself free, he should pinch in towards the middle while remaining on the same level with the ball. This way, he's preserving the compactness of his team as a whole.

In offense, the winger has to stay wide and be aware if there is open space on his flank available. When an attack is initiated on one of the flanks the entire enemy defense usually shifts to that same side. This leaves the opposite flank open. Using this open space, the winger can dribble up the sideline. Beating opponents on the outside creates many problems for the enemy defense. Besides good stamina the winger must also possess good dribbling skills.

Defensive Midfielder

The defensive midfielder is the backbone of the team. His job is similar to that of the sweeper with the exception that he operates in front of the defense, between the offensive midfielders and the stopper. He roams laterally from sideline to sideline, usually pressuring the ball. If any of his teammates gets beaten in the middle of the field, the defensive midfielder is expected to provide backup.

In offense, the defensive midfielder must stay behind the attacking line, collecting rebounds and miss-kicks. He is also expected to

Attacking / Offensive Midfielder

Offensive midfielders are usually the fittest players in a team, considering that they have to be (directly or indirectly) involved with the ball during every attack. Apart from their top form, midfielders need to have good technical ability. When a skillful midfielder is dribbling, he is not supposed to look at his feet but should be scanning the field for open spaces and teammates making runs. In a sense, the midfielder needs to possess enough skill so that he can concentrate on his primary job of distributing the ball without worrying about the mechanics involved.

Striker

The main ability of this player must be his strength and heading.
His job is simple: to stay in front of the enemy goal, attract enemy defenders and possibly score. Unlike the typical forward, the striker doesn’t shift to the sides as much. He stays in the middle as a target man and will typically be marked by at least one player.
On attacks, he may try checking to the ball and shielding/laying it off or would simply stay with the last enemy defender.
His defensive duties are to pressures the opposing defenders
while they are initiating an attack.

Forward

The job of the forward is not merely to head for goal every time he receives the ball. He must also make runs towards his own goal and support the midfield rather than simply rush forward. This can best be observed by watching top level games, where forwards are often playing with their back turned to the enemy goal. Whenever a team is playing with two forwards, they must both move in conjunction. If the ball is on the left flank for example, they must shift towards it so that one of them is in the center and the other near the sideline.