This filter is found in
Out-of-focus photographs but also most of digitized images need correction of sharpness. This is due to digitizing process that must chop a color continuum up in points with slightly different colors: elements thiner than sampling frequency will be averaged into an uniform color. So sharp borders are rendered a little blurred. The same phenomenon appears when printing color dots on paper.
The Unsharp Mask filter (what an odd name!) sharpens edges of the elements without increasing noise or blemish. It is the king of the sharpen filters. It's a pity we don't have any preview!
Some scanners apply a sharpen filter while scanning. It's worth to disable it so that you keep control on your image.
To prevent color distorsion while sharpening, Decompose your image to HSV and work only on Value. Then Compose the image to HSV. This is well explained at gimpguru.org