Protein skimmers have a number of benefits to a reef tank.  They help to expel excess carbon dioxide from the water, assisting in maintaining a more stable pH; they remove organics from the system and allow for keeping a variety of corals species in a confined space and improve visibility and light penetration by removing suspended solids.  Not all reef aquariums require a protein skimmer as many LPS and non-zooxanthellate corals benefit from the excess organic matter.  Lowering the availability of food to filter feeders can reduce the biodiversity of live rock. 

There are several types of skimmer designs: venturi,  aspirator or needle wheel, downdraft, spray injection and beckett.  Each has their pros and cons:

Venturi: a small valve directs the stream of water in a pipe through a very small opening in the pipe.  The pressure differential causes air to be sucked in through ports that open to the outside forming a fine mixture of air and water which is introduced into the bottom of the protein skimmer.  The venturi rarely needs to be replaced but the water must enter under pressure and therefore requires a strong water pump.  These skimmers can be sensitive to changes in their immediate environment such as placing ones hand in the aquarium or feeding the fish.

Aspirator: this system uses an impeller with pegs on it which chop the incoming air into fine bubbles.  This design packs the skimmer witha  high density of fine bubbles making it more efficient.  The pump does not to be as large as it does with the Venturi design. These skimmers are generally smaller in size and quiet to operate but not much water can be processed as in some other designs.

Downdraft: water from the tank is injected through a narrow nozzle under pressure into a column filled with bioballs.  The mixture of air and water creates fine bubbles that create foam. This design allows for large amounts of water and air pumped through the skimmer per hour.  However, this design may be to efficient, stripping the water of its nutrients very quickly.  A large water pump is required and the system can be quite noisy and require a lot of space

Spray injection:  water is forced through a specially designed nozzle that acts much like a spray nozzle on a garden hose. This design allows for a smaller system that operates with a smaller pump.  Fluctuations in sump level can affect performance of the skimmer and it may require constant "dialing in". 

Beckett: this system uses a special nozzle called a Beckett which produces a fine spray.  These skimmers can be as tall as downdraft skimmers but lack the external mixing column.  They do not require a large pump to operate.