A slice is a golf shot that curves off to the right (for right-handed golfers). It is one of the most common mis-hits for amateurs.
Many golfers struggle with a slice because they don't understand the cause.
The physics that cause a slice revolve around two things: the club face and the swing path. When you understand the cause of a slice, you can focus on correcting it.
There are three different types of slices: a pull slice, push slice and standard slice.
The pull slice starts left of the target and slices back to the right. A pull slice will end up the straightest of the three because it starts left. A push slice starts right of the target and slices even further--it is the most detrimental. A standard slice starts down the target line and then goes right.
The sliced shot has more backspin and sidespin than a shot that's properly hit.
Having more backspin causes a slice to be much shorter than a properly hit shot. The sidespin causes the ball to curve to the right in the air and then bounce and roll even further right when it hits the ground.
A standard slice is caused by an open club face and an outside-to-in swing path. These combine to create the sidespin that causes the ball to go right. The more open the club face is at impact, the more sidespin the ball will have. The swing path determines the starting point of the slice. The more a golfer swings from outside-in, the more the ball will start left of the target before slicing right. A pull slice is caused by a severe outside-to-in swing path. A push slice is caused by an inside-to-out swing path with an open club face.
To fix a slice, the golfer has to square the clubface and swing on the correct path. Closing your clubface more during your swing is one way to correct that element.
The golfer must also fix a downswing that comes over the top, causing the club to swing across the ball from an outside-to-in path. Try closing your stance and swinging along the line of your feet to feel the proper swing path.
There are some instances where slicing can be beneficial.
One would be when playing a hole that has a severe dogleg to the right. A controlled slice allows the ball to follow the design of the hole.
You might also want to hit a slice when your ball is in the woods and you have to curve the ball around a tree. In both cases, you want to swing on an outside-to-in path with a slightly open club face.