[Bachs]

Unsharp Mask was a technique used in the old 'wet' darkroom by creating a duplicate negative and making it slightly out of focus ('unsharp') and overlaying it, slightly out of registration with the original to make a print.

You can demonstrate the effect for yourself digitally in Photoshop using the following method:

Use a large resolution image to try this on as the minimum parameter for gaussian blur is 0.1px which will look excessive on the end result in Photoshop if you use a websized image.

I'm not going to post a page full of screengrabs as the technique is simple to follow if you know your way around Photoshop.

Open an image in Photoshop and copy the layer.
With the copy layer selected, go to Image>Adjustments>Invert to create a 'negative'.

With the inverted copy layer still selected, go to Filters>Blur>Gaussian Blur and enter the minimum allowed value of 0.1px and click OK.

From the Blend Menu on the Layers Pallette, select Overlay as the blend method.
(The image may look a little muddy at this point)

Using the arrows on your keyboard (remembering to click outside the blend menu so you don't continue to scroll the blend methods) click the down and right arrows one click each to misregister the copy layer.

Merge the layers and adjust contrast.

You will see the sharpening effect on the combined image.

No doubt Photoshop uses a much more complicated method than the above but it should give you an idea of how USM works in principle.