[scoundrel1728]

The term "unsharp masking" is a legacy from the days before digital photography. Back in those days, a slightly blurred, low-contrast contact positive - the unsharp mask - was made from the original negative. The two pieces of film were then sandwiched together and printed out. High-contrast paper was used to bring the overall contrast back up to its original value, with the edges then developing especially high contrast because these areas were not partially masked by the positive.

A similar effect is applied digitally today, where the radius setting corresponds to the amount of blur and the intensity to the amount of contrast enhancement. Because unsharp masking also tends to emphasize noise, as high-pass filters generally do, the threshold is added to counteract this effect while still enhancing the contrast of the real edges.

The effects of denoising and sharpening are largely complementary. Things done to increase sharpness also tend to increase perceived noise and vice versa. It can be a real balancing act to sharpen a noisy image or to denoise an image without undue loss of detail.