A 'sump' is the general term for any type of container holding water below your tank. Sometimes a sump is a wet/dry trickle filter, or an aquarium fully customized for sump duty.
A refugium is any area designated is a 'refuge' for critters, such as copepods/amiphpods, that can get 'used up' in the main tank. A common benefit to a refugium is you can add various types of macro algae, such as caulerpa or chaeto, to help consume excess nutrients in your system. There are a number of different refugiums including in-tank, a Hang-On-Back style, or the most popular, part of a sump dedicated to a refugium.
The most common modern day sump has 3 basic sections (not including perhaps an intake section). An equipment section, obviously where you'd put your heaters, skimmer, etc. A refugium section, just discussed, rubble rock for pod growth, macro algae, a deep-sand-bed perhaps, and finally a return section, where the pump that would bring the water back to the main tank would go.
Your two filters, unfortunately, will not make the most out of a sump setup. In a display tank with adequate live rock, there is no need for a bio-wheel filter, so your Emperor 400 is definitely not needed. The CPR aeroforce skimmer is also a hang-on-back, which again sorry, it not really the best type of skimmer. There is a sacrifice made with a HOB skimmer, you give up some filtering (skimming) power due to the design of the skimmer having to be a HOB. A skimmer designed to be in-sump are many times better then a HOB skimmer.
