My personal definitions:
A Refugium, as best I can describe the idea, is a "refuge" or safe place for beneficial critters, macros, etc. to grow unhindered by predation. There are many types including Hang On Backs, In Tank, Sump Combos, Seperate Tanks (the one I have), Etc. Refugiums are used for many things including pod growth, nitrate reduction, plankton production, etc.
A Sump is a seperate body of water, usually out of sight, where you can house your otherwise unsightly heaters, skimmers, etc.