Lets say I have two opened images I am working on in Photo Shop. I want to copy an entire layer or a smart object from one opened image to the other opened image.
How do i do it?
For a layer:
In your original image right click the layer in the list and select the "Duplicate Layer..." option.
You get a dialog prompting for the new layer name and (importantly in this case) a Destination drop down. It's defaulted to the current document but you can select another open document or create a new document. If you select another document you can also set it's name.
You just drag from the layers tab, the smart object and drop it to the other window with the second photo that you like to copy.