The scrum-half is a rugby position also referred to as the 'half back'. It is usually the strongest man in the back line as he will spend a large percentage of his time up with the forwards. However his main purpose is to "feed" the scrum, this involves rolling the ball (in a non bias fashion) into the tunnel created by the two opposing front rows then running behind the N.O 8 and retrieving it then getting out to the backs or running it him self.
In Australia, the second row of the scrum are often referred to as "second row", the position behind them as "lock", the forwards on either side of the lock as "breakaways" (some apply the term to the number eight as well as the flankers), and the fly-half as "five-eighth".
Although the scrumhalf may put the ball in on either side of the scrum, he is unlikely to choose the tighthead side because otherwise the opposing hooker would be between him and his own hooker.
The hooker uses their feet to 'hook' the ball in the scrum, because of the pressure put on the body by the scrum it is considered to be one of the most dangerous positions to play.