A fuge is a refuge or refugium, meaning a place of safety. As Biffer pointed outit is a place of safety from fish and organisms that will eat them almost as quickly as they can reproduce. A refugium can have live rock and macro algae and be lit on a reverse cycle from your display tank and therefore contribute to the tank in many ways. The refugium can provide the same functions of a sump plus functions that a sump can not perform. If you have room for only one under your tank then the obvious better choice is to provide a refugium as it is also a sump. But a sump is basically just an empty compartment that people have been also using for skimmers, heaters and such, but a refugium with planning allows for these things also. A refugium can also be a display tank that is fed from the main display tank and has its overflow drain to the main display tank. These can be nice to view when teaming with small critters, live rock and macro algae. This is becoming more popular, but usually does not allow for reverse lighting as well as the under tank refugium. Refugiums can also be turned into pod/zooplankton culture centers with large capacities to produce pods/zooplankton with specific feeding of phytoplankton. To grow specific sizes of pods usually only requires that different sand sizes be used. Large sand for larger pods and small sand for smaller pods. This usually only matters if you have SPS corals as they typically feed on only the smaller pods.