Small glitches and artifacts can be readily removed using the Edit Pixels command. Using this command, you can pick up or select a color, then apply it to one or more pixels.
The Clone tool is much more powerful. You can select any size aperture using the right-click menu. You can then copy data from one region to another. You can copy pixels from one place in an image to another; you specify the source region, and then click on the destination where you want the pixels copied to. As you move the cursor around, the source region moves in parallel. This allows you to copy strips or areas. This is extremely useful for cleaning up areas that have significant background in them. If you can find a source area with the same background characteristics, your repair can be seamless.
The Clone tool also allows you to copy from one image to another. In addition to copying from any area in the source image to any area in the destination image, you can lock the two positions together. This is handy for merging information from two slightly different versions of the same image; for example, you could use this to smooth or sharpen some region of the image. You can use the Undo buffer of the image as the source; this allows you to selectively undo processing operations on certain areas of an image.