Protein skimming is a simple, yet extremely effective way to greatly improve water quality in saltwater aquaria. The principle is that air bubbles attract protein molecules and form a scum. This scum is collected via a cup at the top of a vertical tube for disposal. By removing proteins (uneaten food, excreta, dead material etc) before the nitrogen cycle has begun greatly reduces the speed at which nitrates build up in the tank. In addition skimmers are extremely effective at oxygenating the water allowing for increased stocking levels.
I collect a glass of disgusting brown paste every 2 or 3 days which would otherwise be fouling the water.
The downside of skimmers is that they are also excellent at removing plankton from the aquarium, when feeding soft corals, bivalves and other plankton feeders its a good idea to turn off the skimmer for a couple of hours or so.