Applications
A typical use for the tool is in object removal – more colloquially, "airbrushing" or "photoshopping" out an unwanted part of the image.
If a part of an image is removed simply by cutting it out, then a hole is left in the background. The Clone tool can fill in this hole convincingly with a copy of the existing background.
A common usage is in retouching skin, particularly in portraits; in this way one can remove blemishes (pimples, moles, or scars), and make skin tones more even. Cloning can also be used to remove other unwanted elements, such as telephone wires, an unwanted bird in the sky, and the like.
A more automated method of object removal uses texture synthesis to fill in gaps. Of these, patch-based texture synthesis or "image quilting" is essentially an automated application of the clone tool, choosing the optimal source area so as to patch over with a minimal seam.
In some cases, the undesired object is mixed with the remained of the image, and a simple circular brush, even with feathering, would not work. For these cases, some programs allow that object to be selected by color/outline so other areas are not affected. Other programs allow edge/color sensitive brushes to deal with such objects.
Read more about this topic: Clone Tool