What the difference between an inverted and everted retina?
Based on your title and description, all that means is that in vertebrates the retina is inverted in a sense that light sensing cells sit at the back of the retina meaning light has to pass through layers of neurons and capillaries before reaching light sensing cells, whereas cephalopod has the photoreceptors at the front side of the retina, with processing neurons and capillaries behind them hence retina is everted i.e. inside out. See this page (http://en.wikipedia.org/wiki/Retina) under "In other animals" section.

Hope this helps!