I believe that Safari now takes a screenshot of the page you were previously on, and when you go back, it shows this screenshot at first while the page is reloaded.
The iPhone does something similar when you close an app - you're shown a screenshot before the actual interface is ready to be used, because they've found that people rarely instantly do something - they take a moment to re-orient themselves.
On Safari, though, I found this behaviour to be incredibly irritating and interrupting to my workflow, so I switched to Chrome with retina compatibility.
Image retention / ghosting is specifically when an after-image of what was on your screen is retained, like a faint reminder of what was there. Check out the photo attached to this post, in which a pure grey background is supposed to be shown - that's image retention.