Though almost everyone who plays soccer knows the basics of the soccer out of bounds rule, I have still heard this rule questioned in certain circumstances.
When the ball crosses over the sideline in soccer it is called out of bounds.
When an out of bounds is called, the team opposing the player who kicked it out chooses a player to throw the ball onto the field from just outside the sideline.
When throwing the ball in, the player must keep both feet on the ground and throw the ball with both hands from just behind his head.
Also, if the ball were to go out by crossing the goal line, then either a goal kick or corner kick would take place, depending on who kicked it out (a goal kick if it crossed the opposing goal line of the player, and a corner if it crossed your own; for more detail see how to play soccer).