A protein skimmer is a device used to remove organic compounds which would build up over time and cause elevated or even toxic levels of unwanted nutrients in the water.
There is a little bit of debate as to wether you can "over skim" your water, this means you run a huge skimmer 24/7 in an attempt to remove every last scrap of organic waste.
Intermediate and advanced methods of reef keeping employ this method and then add back the lost nutrients in a more controlled manner.
Until you have a full understanding of what's going on behind this method then it's best to use a skimmer that is rated for your size tank or just above.
Air stone driven skimmers often attract new hobbyists due to the cheap price but stay away as they are ineffective long term and usually end up in a junk pile.
They have a simple design body with an open bottom to which the air stone meets the water and usually a wooden air stone for making the micro bubbles.
Becket injection and downdraft skimmers are an old but reliable technology and they are still being used with great results today.
They utilise a similar system in which very high pressure water is forced through a nozzle that foams the water as it draws in air from a nearby inlet.
The water is forced downward into the reaction chamber and this gives more "hang time" for the bubbles to attract waste particles as they finally rise to the surface and leave clean water behind to flow back into the aquarium.
Commonly available skimmers use the "Venturi" method, it uses a pump with a specifically designed impeller (pinwheel) to suck in both air from an intake tube and water from the main pump inlet and cut up the air into multitudes of tiny micro bubbles.
These MicroBubbles are where organic compounds are bound onto them in a process that involves the waste particle being attracted to the micro bubble and taken to the surface.
The waste continues to collect at the neck of the skimmer and eventually is pushed up and over the top into a collection cup as the waste builds.
Information provided by:MARINE AQUARIUM FANATICS AUSTRALIA