Unsharp Mask Filter
The Unsharp Mask Filter increases small details. It does this by first blurring the image with a Gaussian Blur Filter and then subtracting the blurred version from the original image.
The "Radius" parameter controls the size of the details.
A small radius highlights small details.
A large radius highlights larger details
Strong Unsharp Mask Filters also increase image contrast. To lower contrast again, you can use the "Contrast" parameter. It can also be used for many other effects.
An Unsharp Mask Filter with a very large radius can also be used for automatic color correction.