Refugiums In General: Refugium employment in our hobby today has become increasingly popular helpful to reef hobbyists. Here is a brief list of refugium characteristics and benifits:

a seperate chamber from the main tank but shares the water column...
contains live rock and/or live sand...
can be found with or without a lighting system (see "lighting your refugium")
chambers allow uninhibited organism development free of usual grazers...
refugiums can serve as a method to settle of detritus...
can include sand sifting start and cucumbers to feed on settle detritus...
a method of cultivation of food for your system (example: mirco-crustacean growth)...
growth of macro algaes for natural filtration and food...
a chamber to keep those vulnerable yet benificial reef critters like small shrimp...
could be used as an emergency for acclimation or even breeding...
Macro-algaes going "sexual": I've got a small refugium on one of my reefs and I have a problem once in a while with the various macro-algaes going a white translucent color and then the plant disintegrates. Obviously, I was worried about my water quality and wondered why it died. I have since learned that some macro-algaes go "sexual". What this means is that when macro-algaes goes sexual it turns a white translucent color and releases sperm and eggs into the water and the rest of the plant disintegrates and releases excess nutrients into the tank. It is a way for algae to spread in a much faster way in the wild. In captivity however it can be kind of bad depending how much goes sexual due to the excess nutrient release. You should remove the macro-algaes as soon as it shows any sign of going sexual. I am unsure as to what triggers macro-algaes to go sexual but I've had some success keeping the macro-algaes trimmed frequently. Grape caulerpa (caulerpa racemosa?) seems to be the most common to go sexual with the Spaghetti macro-algaes least likely to go sexual. Some people who use things like phosphate removers or heavy carbon users seem to have their algaes go sexual very soon afterwards. From that I would gather that having trace elements added on a regular would be beneficial and making sure you don't make your water too clean or nutrient free.

Lighting your refugium: I've seen quite a few discussions on this subject. I will start off with the topic saying (personal opinion) that I think lighting a fuge 24/7 isn't the best idea. First of all, algae never sees light 24/7 in the wild and with the variety of life in a fuge some nocturnal critters would be able to live in a more natural state and to add to that a regulated dark/light cycle can help initiate spawning in many critters. Part of me justs want to say that keeping light on some of these critters 24/7 is not at all right because I feel it could stress them out and the lack of a, for lack of a better term, sleep period. My refugium is lite on a reverse cycle from my main tank. A flip-flop schedule has several benifits to a reef tank. One very important one I will mention here is the stabilizing effect on pH. The photosynthesis that will occur in a refugium working on a main tank (utilizing the reverse light cycle) will keep carbon dioxide levels low, keep disolved oxygen levels high, and keep pH stable. On the flipside of things, I have learned that lighting a fuge 24/7 will boost the growth rate of algae that will therefore remove more nutrients from the system at a faster pace. The negative to that would be you would need to trim the growth more often. Your LFS may like you to bring it in to trade. A few extra fish dollars always helps. Another benefit is "...these algae are one cell, the sugars that are made by photosynthesis are used effectively immediately or converted into a starch for later use. Darkness is a stress time for these organisms where the starches made during the day are converted back to sugars for the algae to use to simply stay alive until the light comes back on..." (R.Shimek from a ReefCentral.com thread). Many people have also said that keeping a 24/7 fuge keeps the algae from going sexual. I have seen quite a few refugiums running both in a day/night and 24/7 cycles and it seems that most nocturnal fuge critters just adapt to a constant lighting situation. Need to made a decision on what type of system to run? On a personal observation, I can say that tanks with a heavy load (lots of fish for instance) may benefit from a 24/7 system as the nutrient load may be high enough to necessitate the option. My fuge didn't do well that way as my fish load is low and I have many clams in my tank. The nutrient level is lower and can't support a 24/7 fuge appetite. As you can see there seem to be benefits to both methods and you may want to try both to see what works for you best. I will be adding to this as I learn more.