As David said, a true sump is just another tank which holds more water, maybe your protein skimmer, heaters, etc, and makes for a great place to add supliments and such without having to go inside the display tank.
A true Refugium would be a seperate tank which is also on display which would house more delicate creatures, say a sea horse or such, out of harms way. Then along those lines you might grow some macro algae in there like culurpa which helps to remove nitrates.
The "sumpugium" as I like to call it is something in between, basically a sump with a medium to deep sand bed with some live rock in it, and culerpa. Its not displayed, rather hidden like a sump, and gives you some of the benifits of both systems. In this case it will also be a place for pods and worms to grow, and hopefully migrate back into the main tank, either through the pump, or what I do is swap some of the live rock back and forth at each water change.
-Scott
