In any sport, a foul is a violation of that sport's code of conduct, and each sport punishes foul play in its own unique way. Soccer defines a foul as an unfair action a player commits against an opposing player or the opposing team during the course of a match. The punishment varies depending upon the nature of the infraction, but will be either a direct or indirect free kick. To be a foul, the act must occur on the field, while the ball is in play. Otherwise, it may constitute a misconduct, and may even warrant a caution or send-off, but it will not be a foul.
For some minor infractions the punishment is an indirect kick. This means that at least two players must touch the ball before the kicking team can score. These fouls are often called "technical fouls" because most are not the direct result of foul play, but are violations of some procedural rules designed to keep play flowing or prevent more serious fouls from happening. One such "technical foul" is the offense of impeding an opponent – commonly known as "obstructing."
Impeding an opponent
Soccer players often get in each other's way during the normal run of play. Sometimes, though, players will intentionally block their opponents from running after the ball or moving into tactically important space on the field. The offense of "impeding an opponent" involves the deliberate use of the body to interfere with the other player's movement in order to delay his progress, and is punished with an indirect kick. This foul often occurs when a player senses that a speeding opponent will beat him to the ball, or anticipates a potentially troublesome pass into open space. By stepping in front of the opponent and blocking his progress, the player hopes to slow his opponent down so that a teammate can collect the ball. If this action causes more than incidental physical contact with the blocked player, it may constitute a "charging" or "holding" foul, rather than "impeding."
A player within "playing distance" of the ball may legally block his opponent's pathway to it without being guilty of a foul. This is known as "shielding," and involves the use of the body to keep possession. Playing distance will vary depending on the speed of the players and the ball, and ultimately rests with the referee's judgment, but the shielding player does not actually have to touch the ball to shield it legally from the other side. Shepherding the ball out of bounds or towards the keeper is a recognized and quite legitimate tactic, as long as the player guarding the ball stays within playing distance.