If you're referring to an algal turf scrubber or ATS, there are lots of methods. Try a Google search on "ATS" or "Algal Turf Scrubber" or a book called "Dynamic Aquaria" by Adey & Loveland. Walter Adey was fairly successful with algal fitration at the Smithsonian. The overall effectiveness vs other filtration methods is debateable (isn't everything) but Delbeek & Spung go into it a little in TRA.
Essentially, you run your water over a algal mat (usually a screen with algea growing on it) which is lit. Plankton will grow in this mat and can eventually end up in the display tank feeding it. Every now and then you harvest some of the algae mat as nutrient export and either toss it or feed it to fish.
Algae can be most any variety from hair to combinations of Caulerpa and others. Lighting can be anything from fluorescent grow lights to used MH. Many folks use spray bars to sray water on the algal mat, I use ramps. Size can range but I find 24" x 12" (or so) to be a fairly nice size, easy to manage, and not expensive. Any old fish tank can be made into one or buy one pre-made with all the fixin's.
IMO, refugia are fairly similar just a little more passive.