A throw-in is the way to restart play when the ball leaves the field by completely crossing over a touch line.  The team which did not touch the ball last takes the throw-in.
A throw-in must be taken where the ball left the field (within a yard of this spote).
A thrower stands off the field but a part of either or both feet can be on the field if at least some part of each foot is on the touch line or off the field
The ball is thrown over the head using both hands while the feet remain on the ground.

The ball is in play as soon as it is released from the thrower's hands and enters the field.
If the ball does not enter the field or hits the ground before entering the field, the throw-in is retaken.

If the throw-in is not performed correctly, the ball is given to the other team for a throw-in.
The main purpose of the throw-in is to get the ball back into play
A goal cannot be scored directly from a throw-in

If the ball goes directly into the thrower's own goal, play is restarted with a corner kick

If the ball goes directly into the other team's goal, play is restarted with a goal kick
The thrower cannot touch the ball again until it has been touched by another player (indirect free kick for the other team)
An opponent who distracts the thrower or interferes with the thrower must be cautioned and shown the yellow card for unsporting behavior

Examples of this might include an opponent jumping in front of the thrower or waving their hands about to prevent the throw from going in a particular direction