I'm sleepy, so it was a bit wordy, but basically, it's a stand alone image that you can save and transplant from image to image and reuse. You can make them by isolating images from their backgrounds, or you can create things like Poser renders and save them in format that either has none, or that has an alpha channel so you can remove the background with a click of a button.

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi