Once a photo has been corrected for contrast, brightness, and color, a number of filters can be applied to smooth out color, as well as to cover scratch marks and spots.

The photo at right is being enhanced by the Unsharp Mask filter, which smooths out color differences by blurring and then sharpening the edges.

In the picture below, the Clone tool, or Rubber Stamp, (seen in the toolbar at left,) has been used to even out skin tones. It does this by selecting a nearby area and recreating the texture and color. Airbrush and smudge can smooth out rough areas as well.

Many photo editing programs also include filters and effects such as adding text, lighting effects, and filters which can completely change the way the picture looks.
But that's another website!

Happy imaging!