The radius part of the unsharp mask is really the radius for a gaussian blur, hence the name "unsharp". What happens is photoshop blurs a copy of the image, which has the effect of destroying edge sharpness. Then, the unsharp version is compared with the original to see how much the pixel value changed. If there was an edge there previously, the difference between the unsharp and the original will be significant. If there was not an edge, there will not be much change. The threshold is the amount of difference needed between the original and the unsharp to change the value of a pixel. The amount controls how much the pixel is changed lighter or darker, based on how much of a difference there was between the original and the unsharp versions. Keep in mind this happens in each channel individually, so all caculations are done in black and white.