Wet / Dry Filters are a part of some sump filter setups.  You will find that there are several methods of setting up and filtering a marine aquarium.   From what I've observed, nearly every marine hobbyist has his or her own opinion on what works and what doesn't work.  I took a ton of opinions into consideration, and originally decided on a wet / dry as part of my marine filtration system.  Presently, I DO NOT USE a Wet/Dry filter.  Read on...

A Wet / Dry Filter relies on biological filtration, that is, after the system cycles, the wet / dry actually houses a colony of  bacteria that naturally breaks down the wastes present in the aquarium.  Ever walk along a creek or river, and almost slip on a slippery rock?  That rock was covered with microorganisms that feed on animal wastes, and break it down into simpler forms. 

A typical Wet / Dry Filter starts with a layer of filter material, which removes most of the particles that may be suspended in the water.  The water passes through to the next layer, usually filled with Bio Balls or similar media.  This media increases the surface area that the water can pass over, thus encouraging more bacteria to form.   This is the "dry" part of the filter, because the water simply "trickles" through the media.

Next, water accumulates in a tank or sump.  This sump may contain several chambers to "trap" sediment, or allow for a chemical filtration chamber.  Lastly, the water is pumped back to the aquarium to complete the filtration.  It is very common for hobbyists to include other hardware in or off of the sump, such as heaters, skimmers, Kalk Dosers, Chillers and other pieces of equipment used to further ensure a clean, balanced system.