OK so here is an article about wet dry filters. Here is an article on how wet dry filters work..
http://saltaquarium.about.com/cs/filtration/a/aa090298.htm But a wet dry filter is basically just a large mechanical filtration device, the water goes through floss which then goes over bio-balls (this is a whole area of contention in todays reef keeping world) or live rock rubble which is then collected in area and returned to the tank.
A refugium is a biological filtration area, it is normally part of sump system and again normally contains live sand, live rock and macro algae's that are used for waste conversion/denitrification/nutrient export.
As far as putting a valve to restrict the flow from the pump I wouldn't for anything other than emergency water shut off. It will make your pump work harder, make it heat up more and shorten its lifespan. If you can T it off after it pumps out and then put like a ball valve there to adjust the flow going into the t vs back to your tank and equalize the flow rate. I have a valve on mine going to the tank but like I said its for emergencies only, otherwise it stays completely open.