Cloning is the process of duplicating samples from one part of an image to introduce replicated content for creative effect or to repair the original.
The Clone Brush Tool copies pixels from one part of an image (or layer) to another. The tool uses a source (shown as a '+' cursor) to clone from; this moves in relation to the tool's cursor (shown as an 'o' cursor) with its position being able to be redefined as you clone from different areas.
The clone source can be one of the following context toolbar options:
You can define a global source in a secondary open document and paint the sampled pixels into your working document by selecting the clone source option 'Global'. The Sources panel stores all your global sources; you can then select any stored global source and clone from it. Any new documents can make use of the panel's global sources.
The Healing Brush Tool paints samples from one part of an image onto another. It's useful for removing defects and for general photo retouching. In many respects it works like the cloning, however, it blends the target pixels with the sample pixels by matching the texture, tone, and transparency of the sample pixels with the target pixels.
The source is stored, along with other global sources, in the Sources panel. The panel can be switched on via View>Studio.