Cloning Brush Tool
 

The cloning brush is an editing and photo-correction tool that paints one part of an image over another part of the same image or another image in the same document using a user-defined fixed offset. It thus enables you duplicate objects by copying them to other parts of the image or to remove defects by patching them over with unaffected parts. Holding down the left mouse button clones; right-clicking with the mouse (or left-clicking while holding the Shift key) sets the cloning offset (source point).

As explained above, you may use this tool to copy objects from one part of an image to another, as shown below (the second aeroplane is a cloned copy of the first, flying two jet-liners that close to each other could cause a crash):

You may also use this tool for photo-corrective tasks like removing pimples by way of patching the affected areas with pixels from unaffected areas, although the Healing Brush is the preferred tool for that task:

You can set the size of this tool’s tip by setting the width entry on the toolbox. The level entry is currently not used. This tool is pressure sensitive when using a graphics tablet; the pressure varies the width of the tool tip.

You can select a brush-tip mode for this tool.

It is recommended that you use a soft-tip while cloning.

The cloning brush is a very powerful photo-correction tool, and as such the mastery of its usage is a skill every image editing practitioner, professional or otherwise, should have. Take time to practice with this tool on several photos. It'll be worth the effort. Try it out!