Get out your text-books. I think I can expand on Pauly's theory with this:
General defensive tactics
All eleven players on the pitch have a defensive role. Which roles they are assigned, depends on tactics. In principle, there are two ways of defending, zone defense and man-to-man defense. In a zone defense, defensive players mainly move in relation to each other, whereas a man-to-man defense mainly moves in relation to opposing players. Whenever defensive players are given, or give themselves, a larger degree of freedom, hybrids of the two are seen.
First, second and third defender
The first defender has the main responsibility for what the opponent in possession does, seeking to obstruct dangerous passes. The first seconds after the team has lost the ball are important, as the defending team in these seconds will be poorly organised defensively. Wise first defending will contribute to the defending team managing to organise before the opponents may attack.
The first defender should usually be the player closest to the opponent holding possession, but needs to be at the defending side of him in order to do an effective job. He should keep a distance of about 2 metres, although the ideal distance will vary with each situation. The point is to pressure the opponent as much as possible without giving him a large possibility of a dribble. As a dribble isn't as dangerous when the defending team is well organised, the distance may be shorter in these cases. Analogously, the distance should be increased if the defense is poorly organised.
In certain cases, the first defender should attempt a tackle. Often, however, this will increase the probability of being dribbled and passed.
The direction in which to move towards the opponent with possession of the ball may be the shortest direction. However, it may be of value to curve the defensive run, in order to channel the opponent in a certain direction. If the defensive team is well organised, he should be channeled towards the centre of the pitch. In the case of temporarily poor defensive organisation, however, he should be channeled towards the line.
The second defender is for security if the first defender is passed. In that case, he takes over as first defender, and ideally one of the third defenders takes over as second defender. The team should be organised in a manner to make this transition as fast as possible. The typical ideal distance between the second and first defender is about six metres, but this will vary strongly from situation to situation. The most important factor is the opponent's speed. If he's moving fast, the distance should be longer. If he's standing still, the second and first defender may in some cases join forces and work as two first defenders.
While the role of first and second defenders are rather similar, the third defenders' role is very different in zone defense and man-to-man defense. Also, their organisation will vary with formation.
Pressure lines in zone defense
In zone defense, second and third defenders and midfielders are organised in two lines, in the transverse direction of the field, thus organising a defender line and a midfielder line, the midfielder line working as an "outer shield" for the defenders. The lines should be as straight as possible, although the first defender and in some cases the second defender may rush out of it to pressure the opponent with the ball. A straight line of defenders may prevent spaces behind some of them due to the offside rule. Also, even in zone defense, some opponents, for example those moving into dangerous space, may temporarily need to be marked. The man-to-man defense ideology holds that almost all opponents need to be marked at all times, although they will have to keep an eye on zone considerations as well, and usually a sweeper will be given a free defensive role. In practice, however, every defense will be a mix of zone defense and a man-to-man defense, although often with heavy leanings towards one or the other.
The number of players in the defender and midfielder lines is given by the number of football formations. Some formations use midfield anchors to stop attacks between the two lines. Attackers usually also play a role in pressurising defenders, in order to give them less time to find good passing alternatives.
The lines should be shifted sideways depending where the ball is. The third defenders should keep a sensible distance to each other, depending on the width the opponent's attacking players are distributed in.
Depth considerations
The distance between the defender line and the midfielder line should be kept constant at about 15 metres. However, the defensive line should back up and thus increase this distance, stand off, when there is no pressure on the opponent in possession, as this increases the possibility of a through ball. With tough pressure on the opponent, the distance may be reduced to below 15 metres. Also, as opponents move in close to the penalty area, the defending team will be forced to move their midfielders ever closer to their defenders.
When organised, the defending team will offer no resistance until the attacking team has advanced to a certain height on the pitch. The pressure height, or at which depth the midfielders should start acting as first and second defenders, depends on a lot of factors. For example, as higher pressure is more tiring, it demands players with good stamina. In general, a defensive-minded team will tend to stay lower, thus diminishing defensive risks as opponents get less space. This however, also gives them a longer way to the goal in the event of a break and counter-attack, making the long through ball a typical alternative.
Defensive play in man-to-man defense
In a man-to-man defense system, a 'sweeper' is a central defender who has a free role, i.e. has not been assigned a player to mark. He sometimes takes up a position slightly behind the other defenders, as his defensive role often is to 'sweep up' any attacks that break through the defence and as such he adds valuable depth to the defensive unit. Usually the sweeper will be the controller of the defence. They will determine where the back line should be at any given time. Zone defense does not require a sweeper role, and as many teams have changed their tactics to this, sweepers are today rare.
If you are defending someone, wait for the ball to be passed to them, time your run so that you reach the ball, just as it is about a foot away from the intended receiver. If the person you are playing against is good at soccer (football), they will get their body in the way. If this is the case, give them some room, about a yard, and try to prevent them from turning. Always remain goalside (with yourself closer to the goal than you opponent, so that they can not score.
Roman and Avram sit up and pay attention. You better be writing this down. There is a test tonight.