<a href=showthread.php?s=&postid=11250533#post11250533 target=_blank>Originally posted</a> by Konadog
A sump is usually used to house equipment (skimmers, reactors) and has a high flow of water.
A refugium is a place that usually contains LR, macro algae and is used to harbor small animals like brittle stars, pods, and the occasional fish on time out. It will also have a lower water flow rate.
A mud tank is about the same as a refugium, but uses a type of mud to act as a substrait for worms, algae, and also to act as a filter for the tank.
That's the 5 cent explanation, there are more detailed explanations here if you like.