What happens when you copy a gallery/image to another gallery?

When you copy a gallery or an image to another gallery, that gallery or image will now appear in both the original gallery and the gallery to which you copied it.
Don't worry, it's really the same photo displayed in two different places; you are not using twice as much disk space.

What happens if I delete the copy of a gallery or image?

For Images If the image lives in more than one gallery, only the image in the current gallery will be deleted. For Galleries If you copy a gallery to a different parent gallery, there is still only one gallery, just two paths to it. So, any changes you make will show up in both places. Let's say you have gallery C, which has two paths to reach it - one from gallery A and one from gallery B. You want to remove C from B: Edit gallery B. Select the checkbox by the listing for gallery C.... (more)

Are there actually two copies of the image?

It is more like there are two paths, or ways to view the image, so you are not using twice as much disk space.

What happens when I make a change to one copy?

Because there is actually just one image with more than one path to get to it, when you make a change to the original, that change is also applied to the copy, and vice versa.