What is the purpose of a protein skimmer?

A Protein skimmer is a device that take out protein from the water. Meaning, it takes out desolved solid wastes. The ocean has it's own protein skimmer built in: waves hitting the beach. In some more polluted areas you can literally see all the bubbles along the shore or beach.

For a reef tank you must have one. For just a fish tank, it is highly recommended to have one.