Definition Please... ?

sump.....basic tank, usually below your display or in the basement. Used to add water volume to your system to keep things stable. Some people run filter floss and bioballs in it. Another place to put your heater and skimmer away from the display.
refugium....Better than a sump in my opinion. It offers all of the stuff a sump does except you place macroalgae, sand and rock down there vs filter floss and bioballs. Bioballs can catch an excessive amount of detritus and cause nitrates to not break down. Filter pads can catch a lot of detritus too and build up nitrates unless cleaned regularly. Macroalgae will help remove nitrates and phosphates, and house trillions of pods. Same with the sandbed, there are no fish to eat the pods so they thrive. A main advantage of having a fuge is placing a light over the tank and setting it to come on when the display lights go off. The constant light will help maintain a stable ph during the evening hours.

Both are easy to build, but a fuge can be a little trickier because of the sand chamber must be separated with glass or acrylic from the return pump so the sand doesn't ruin the pump.