a refugium is basically a place to grow macro algae. This algae will convert nitrate to nitrogenous gas which will compleate the cycle in you tank. I belive they also take phosphate out of the water.
I would say most peoples refugiums are either a tank or rubbermaid container kept under the tank. An overflow sends water from the main tank down into the refugium and a return pump will send it back up into the main tank.
there are other ways to have a refugium though, I have heard of people who just take an aquaclear 500 powerfilter, remove the media and grow the algae in there. you have to light it of course, so I'm not sure how you would do that.
Hope I gave you some idea of what a refugium is, don't be afraid to ask for more specifics though
