A slice is a shot with a severe left to right curve. It will typically start down the intended target line before taking a wicked turn to the right. As you can see from the diagram below, it is unlikely that anything good will come from a slice.
For left handers, a slice moves in the reverse: right to left.
The slice, also known as a “banana ball,” is probably the most common mis-hit in golf.
At it core, a slice is caused by an open clubface at impact with the ball, which imparts a clockwise spin. The precise cause of that open clubface, however, varies from golfer to golfer. A slice may be the result of an outside to inside swing path, hands getting ahead of the body, or simply a failure to turn the hands over through the swing.
The slice is similar to a fade, which also starts left and returns to the right, but with less severe consequences.
Attempting to cure the slice is a major industry in golf. I am absolutely certain that without the preponderance of the slice, no golf professional would be able to make a living.