The Unsharp Mask filter performs a better job than the standard Sharpen filter by applying the sharpening selectively to a blurred version. Unsharp Mask tends to leave noisy sections of the image untouched.
Sharpening is a very desirable effect on blurry images or those which have been rescaled. Things are naturally sharp in your vision.
Note that the Unsharp Mask, like all other sharpening methods, will produce an aura around the edges if set too high.
Unsharp mask increases the apparent sharpness of image by increases the contrast around edges. Dark patches near lighter areas get darker, and light patches near dark areas get lighter.
Radius: The radius of the blur before the filtering. Where a value very low (0.1) will get similar effects as the standard sharpening filter, setting this higher removes noise and increases the contrast of the sharpening. Setting the blur radius too high, however, will have an effect similar to the Posterize tool.
Amount: The amount of sharpening to apply to the image. A useful value other than the default is 0.35, where it's usually not enough to produce a significant aura on edges.
Threshold: Helpful when the picture you are working with is very noisy. To avoid sharpening of noise in the image you can set the threshold level higher.