"So what the unsharp mask filter does is, it actually blurs details of the image using Gaussian blur within unmasked areas of the image (the masked out areas would be edge details inside your image). Anyway, now that we’ve got the technical jargon out of the way, what you need to learn in order to start sharpening your photos is what’s coming next.
Once you access the unsharp mask filter, a dialog box will pop up. This box has a preview image inside, and what many people might not actually know is that if you click with your mouse on this preview image you will see the before effect and if you release you can see the after effect inside that same preview image box. Of course you can also just check and uncheck the preview image check box, but I thought I’d let you in on this little neat trick. Plus, checking and unchecking the preview check box would only apply the before and after effects on the image in the main window."
Unsharp Masking identifies pixels that are different that surrounding pixels (you can set both the amount of difference and radius of the pixels that will be compared). Photoshop will then increase the contrast of those pixels, again by the amount you specify.