A slice is a big ol' curve to the right (for a right-hander), and is one of the problems that recreational golfers struggle with most. With a slice, the ball often starts out left of the target line before sweeping back right and winding up well right of the target.
Diagnosing the Slice
Your hand or hands, especially your left hand, may be turned too far to the left. The "V" formed between the knuckle and thumb on both hands should point between your right shoulder and right ear.
The shoulders and/or the feet are often aligned too far to the left of the target line.
The ball might be placed too far forward in your stance.
You may be taking the club back too far to the outside, pushing the club away from you. This often goes along with the club "laying off" (pointing left) at the top. Additionally, there can be a clockwise twisting of the club during the backswing.
Your right shoulder might be going too much out and not enough down. The arms are often pushed away from you at the transition, causing the club to approach the ball from outside the target line. There can also be a "blocking" of the wrists through impact, preventing the club from turning over.