Unsharp masking is a technique for enhancing details in digital images. The assumption is that step changes in the original image are softened slightly due to scanner or CCD non-ideal behaviour.
The process starts by further blurring a duplicate of the image, then "subtracting" this blur from the original. The theory here is that (if you get the numbers right) the blurred image is further along the same continuum of degradation, so the subtraction should get you closer to the optical original.
Of course, there's a lot of assumption, and you can't generate correct information that isn't there, but with careful use, the unsharp mask can provide natural-looking enhancement to most digital images.
Most implementations of unsharp masking (GIMP, Photoshop) give you numerous parameters to play with: