Despite being one of the shortest of the seventeen Laws of football, the offside rule, Law 11, is probably the most misunderstood of all. The law dates back to secondary schools in the 19th century, which created it to stop players "sneaking" near the opposing goal and waiting for cheap Soccer Jersey. The offside rule has changed several times since then to adjust the pace of the game, but the purpose remains the same. Most recently, FIFA adjusted the rule in 2005 to avoid offside penalties for players not involved in the current play.
A player can only be offside if they are on the side of the field that contains the opposing team's goal. The purpose behind the offside penalty is to prevent attackers from waiting too close to the goal. You are on the opposing half of the field if any part of your head, torso, or legs are over the line. Arms and hands do not count. A player can only be in an offside position if she is between the ball and the opposing goal. The attacker is "onside" as long as there are at least two defenders level with him or in front of him. If there are one or zero defenders between the attacker and the goal, and the attacker also meets the two conditions above, he is in an offside position. There is no penalty just for being in an offside position. The referee checks an attacker's position only when one of their teammates touches the ball. As soon as an attacker releases a pass, the offside and onside status of each teammate "freezes." Each player will remain officially offside or onside no matter where they move. This only changes when the ball touches another teammate (causing offside to be "calculated" again), or when an opponent makes a deliberate play with the ball (removing all offside statuses).
This is why you often see attackers sprint past defenders as soon as the ball is played. Even if someone is past the defender when she receives the ball, she is still considered onside if she was behind the defenders when the ball was released.