Sharpen filters bring out detail in images by increasing the contrast of pixels next to one another. More advanced image editing programs offer several options such as Sharpen, Sharpen More, Sharpen Edges and Unsharp Mask (USM). Unsharp Mask gives you a lot of control over how an image is sharpened.
Sometimes a photo will benefit from selective sharpening. You select an area with a programs selection tool and only sharpen the area. The important thing is not sharpen an image too much.
