The clone stamp is such a great tool because it lets you copy from one area of a photo to another using any type of brush. This can be useful for tricks such as covering up blemishes (by copying from another part of skin) or removing trees from a mountain view (by copying parts of the sky over them).
To select the area you want to copy from, move your mouse to the area you want to duplicate and Alt-click (Windows) or Option-click (Mac). The cursor will change to a target. Click the exact spot you want to start copying from.