Adding and Removing content
Now this is pretty much without limit.
Adding pretty much boils down to matching the lighting/quality/colours of the original image.
Subtracting less than full frame elements is a tiny bit more work than adding, but as long as there is something nearby that you can clone/heal from it isn't too bad. Removing background is quite easy - green screening makes it easier still, but as long as there is some contrast between foreground and background it's a quick job regardless.
Sharp, defined edges make both of the above much easier, but if one is willing to put forth the time/effort - hair and other things are possible. They are constantly improving the "Edge Defining" of Photoshop. Until that is perfected though, a rough edge and some post work blurring and smudging makes more sense than fiddling with sliders (contrast/feather/radius/expand/contract).
Moving picture elements is basically subtracting, then adding without as much correction for light/colour needed - depending on the shot.
In this quick video, I'll show an object being removed via the clone and heal tools.