The Sharpen tool focuses soft edges in a photo to increase clarity or focus. Sharpening enhances the definition of edges in an image. Whether your images come from a digital camera or a scanner, most images can benefit from sharpening. Oversharpening a photo gives it a grainy look.
Unsharp Mask is a form of image sharpening that amplifies high-frequencies in the image. It is implemented by scaling pixels away from the average of their neighbors.