It should also be noted that in football a player cannot be offside from a throw on.
There is often a great deal of controversy over the offside rule. Attackers will always aim to get ahead of the defenders once the ball has been played and so they try to be as far forward as possible at all times. Frequently a player will have one foot onside and one foot offside. The recent trend in the premiership, encouraged by the FA, is for linesman to signal these tight decisions onside to improve the flow of the game and encourage exciting attacking play.
Linesmen have a difficult job and offside frequently results in a goal being disallowed after it has been scored as it can take time for a referee to notice the linesman's flag (the linesman has no whistle, they simply raise a flag to signify offside, the referee is not forced by the rules to act on this but they nearly always do as the linesman has the best view). As with the current world cup this can cause a great deal of anguish, Italy have had four goals disallowed by linesmen so far. Two of them incorrectly. There has been some debate about the officials in the world cup. Every nation playing has at least one official at the tournament. However for countries like Saudi Arabia and Cameroon these officials may have experienced very little, if any, of high level games.
The best way for an attacker to avoid being offside is to start from a position that is clearly onside and be running to receive a ball ahead of the defence. In these cases the defenders can often be left static and unable to catch up with the already moving attacker. Arsenal's style of play has taken advantage of this with players like Thiery Henry making runs through the defence and getting on the end of a long pass. To combat this teams are often forced to play very deep, as they have done against Liverpool, to counteract the pace of attacking players. This means there is less space behind the defenders for the attacker to run into and it is often easier for a defender to catch up and get back.
One will often see the Offside Trap being played by a defensive line. The aim of this tactic is to catch attackers offside by advancing just before a pass is played to leave the attacker offside. This is a very difficult tactic to play and requires only a single mistake for an attacker to slip through and score. A central defender will often lead the advance but the whole defence must be aware, it only takes one player to play someone onside.
There is also an offside rule in rugby union. This states that,
"In general play a player is off-side if the player is in front of a team-mate who is carrying the ball or in front of a team-mate who last played the ball."
However being offside does not necessarily result in being
penalised. If a player simply moves back onside without attempting to interfere in play in anyway then no action is also taking. It is also possible for a player to be brought back onside by the movement of the person in possession of the ball. It is common for a player to get offside, particularly during
rucks, and they will normally put their hands in the air to signify they are not playing a part in the game and move back onside.
The offside rule also plays a part when the ball is kicked. If the ball is kicked by a player some of whose team mates are in front of him they are all offside. He can play them onside by running in front of them to where the ball has landed. This is the reason that backs, when clearing the ball, will always run up to where it has gone into touch or landed.
The full rule can be found at, http://www.rfu.com/index.cfm/fuseaction/RFUHome.Refereeing_Detail/StoryID/382