Refugium or Wet/Dry

OK, not sure if I should put this here or in another spot, but figured it's a "newbie" question.
I'm trying to understand what a refugium is and what the advantages and disadvantages are compared to a wet/dry (or if these are even performing the same service and thus comparable). I honestly know nothing about the refugium. Thoughts anyone?

 

I'll be the first to tell you what I know, Then the experts will tell you all the right stuff ;D As far as I am aware there are two (2) ways to look at a refugium.
1.) Refugium is meant to be a "refuge" for plants and other critters big and small that you do not want in your main tank. This "smaller" tank is either hung on the side of your existing tank or placed below your tank like a wet/dry. It gets its water from the main tank thru a gravity drain (overflow) and has has its own lighting system and pumps to return the water back to the main tank.
2.) The other use or thought is to put it down below like a sump and to use it as a filtration system for your main tank. It would contain maybe a filter pad, lots of plants, macro algea and other things to help breakdown the ammonia, remove phosphates and other harmful biologicals. It could contain fish or inverts but it is mostly setup as a filter type system again with its own lighting and pump ssytem to return the water flow to the main tank.
Read some of the threads about refugiums and you'll see what I mean on the different schools of thought.
I just realized I didn't answer your question about the advantages or disadvantages. I'm sure someone else will. I'm just getting the thread started  :
Happy Halloween,
Jim