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Yes, the name is backwards. The "Unsharp Mask" tool in Adobe Photoshop and other programs does exactly the opposite of what the name implies--it sharpens pictures. But, it does it by emulating an old photography sharpening trick. It also happens to be one of the best tools in the photo editor's drawer.
Darkroom photographers discovered a trick for making pictures appear sharper. They would start with the negative they wanted to print, and would develop a positive of the same image. When they went to print the negative, they would shine the image through both the positive and a sheet of glass. The end result would be that the image would "stand out" more.
Photoshop does almost exactly the same thing, only with pixels instead of glass. Inside the computer's memory, it produces a negative image of the one on the screen. Black dots in your picture are white in the negative. It blurs this negative, just a little bit, and then it compares the two images, pixel by pixel. In areas where there isn't a lot of difference (like sky), the unsharp mask doesn't make any changes. But, in areas where there's plenty of detail, the original is going to be very different from the blurry negative, and that's where the program goes to work. It makes slight changes to the pixels between areas of different colors, making those borders "stand out" and, in effect, making the picture look sharper.
The three controls for the Unsharp Mask program control how much of an effect it has on the picture. Amount will differ with each picture. A Radius bigger than .8 starts to affect pixels away from the actual edge, so it's a good idea to keep this number low. And you'll probably want to keep Threshold as low as possible, too, because anything over zero affects the "grain" of the image.
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