A hook is the opposite of a slice; the ball curves greatly to the left (for a right-handed golfer). The ball often begins right of the target line (as in the illustration) before sweeping back to the left and winding up well left of the target.
Diagnosing the Hook
Your hand or hands, especially your left hand, may be turned too far to the right. The "V" formed between the knuckle and thumb on both hands should point between your right shoulder and right ear.
The shoulders and/or feet are often aligned too far to the right of the target line. Ball Position
You might have the ball too far back in your stance.
You might be talking the club back too far inside, pulling away from the target line too quickly. This often goes along with the club going across the line at the top. Additionally, there can be a counter-clockwise twisting of the club during the backswing.
Your right shoulder might be going too much down, often with a sliding of the hips toward the target. This causes the club to swing too much to the right through impact.