There has been talk latley about the benifits of sumps as the filtration of marine tanks. There are two main reasons fish keepers use sumps, they are to increase the tanks volume (making water parameters easier to control) and to hide vital tank equipment from the main tank (such as heater, protein skimmers etc). Other uses for sumps include housing the refugium which is as the name says a refuge for: bullied fish, copeopods, amphiopods, shrimps, snails, coral frags and other organisms which would normally be eaten or damaged within the main tank.
The basic of marine sumps work in this order: Syphon from tank -> Bubble trap (3 pieces of dividers which eradicate micro bubbles caused by syphoning) -> refugium -> Bubble trap -> pumps -> return to tank. This method uses the refugium as another source of filtration.
The best filtration within a sump is biomass, refugiums house both organic sand (worms/organisms in sand) and liverock which is covered in benificial bacteria. As long as there is sufficient water movement over live rock this will be the best option for filter material. FIlter material to avoid is sponges, although they can work if properly maintained once their bio mass is reached they will start to produce high levels of nitrate which is of course highly toxic in a marine tank.
Personally within my tank my water flows through the refugium (filled with deep sand bed and liverock) then skims onto a thin layer of sponge then through bio balls to the pump. The sponge MUST be cleaned every 2-3 days and is only used as a catchment for any solids that may pass through. Other ways to reduce solids is to use a bucket trap on the IN part of the sump, this is just a container filled with fine filter material that can be easily accessed and cleaned.