The Hook Loop is a sidespinning loop that curves inward. For a right-handed player, a hook loop curves to the left. The opposite of a Hook Loop is an Inside Out Loop which curves outward in the other direction (to the right, for a right-handed player).
Hook loops are very useful when you want to place your shot out of the opponent's reach. A well-placed hook loop will catch your opponent out of position and unable to get to your loop. The topspin-sidespin is generated by dropping the wrist, hitting on the outside of the ball.
These are sub-pages of the Hook Loop page (a child in the hiararchy).